Genetic analysis of the O7-polysaccharide biosynthesis region from the Escherichia coli O7:K1 strain VW187

We recently cloned biosynthesis genes for the O7-lipopolysaccharide (O7-LPS) side chain from the Escherichia coli K-1 strain VW187 (M. A. Valvano, and J. H. Crosa, Infect. Immun. 57:937-943, 1989). To characterize the O7-LPS region, the recombinant cosmids pJHCV31 and pJHCV32 were mutagenized by transposon mutagenesis with Tn3HoHo1, which carries a promoterless lac operon and can therefore generate lacZ transcriptional fusions with target DNA sequences. Cells containing mutated plasmids were examined for their ability to react by coagglutination with O7 antiserum. The LPS pattern profiles of the insertion mutants were also investigated by electrophoresis of cell envelope fractions, followed by silver staining and immunoblotting analysis. These experiments identified three phenotypic classes of mutants and defined a region in the cloned DNA of about 14 kilobase pairs that is essential for O7-LPS expression. Analysis of beta-galactosidase production by cells carrying plasmids with transposon insertions indicated that transcription occurs in only one direction along the O7-LPS region. In vitro transcription-translation experiments revealed that the O7-LPS region encodes at least 16 polypeptides with molecular masses ranging from 20 to 48 kilodaltons. Also, the O7-LPS region in VW187 was mutagenized by homologous recombination with subsets of the cloned O7-LPS genes subcloned into a suicide plasmid vector. O7-LPS-deficient mutants of VW187 were complemented with pJHCV31 and pJHCV32, confirming that these cosmids contain genetic information that is essential for the expression of the O7 polysaccharide.

their ability to react by coagglutination with 07 antiserum. The LPS pattern profiles of the insertion mutants were also investigated by electrophoresis of cell envelope fractions, followed by silver stining and immunoblotting analysis. These experiments identified three phenotypic classes of mutants and defined a region in the cloned DNA of about 14 kilobase pairs that is essential for 07-LPS expression. Analysis of f-galactosidase production by cells carrying plasmids with transposon insertions indicated that transcription occurs in only one direction along the 07-LPS region. In vitro transcription-translation experiments revealed that the 07-LPS region encodes at least 16 polypeptides with molecular masses ranging from 20 to 48 kilodaltons. Also, the 07-LPS region in VW187 was mutagenized by homologous recombination with subsets of the cloned 07-LPS genes subcloned into a suicide plasmid vector. 07-LPS-deficient mutants of VW187 were complemented with pJHCV31 and pJHCV32, confirming that these cosmids contain genetic information that is essential for the expression of the 07 polysaccharide. Lipopolysaccharide (LPS) is a major component of the outer membrane of gram-negative bacteria (21,24). Its most external moiety, the 0 side chain, varies considerably among bacterial species in terms of chemical composition, structure, and antigenicity (19,24). The 0-side-chain genes in Escherichia coli, Salmonella spp., and Shigella spp. are clustered in the rIb region of the bacterial chromosome next to the histidine operon (21). In Salmonella typhimurium the rfb region was cloned, and specific genes were localized by reference to known deletion mutants (4,5,37). In the case of S. dysenteriae type I, a plasmid-mediated function is required in addition to chromosomal genes for the biosynthesis of the 0-side-chain polysaccharide (30). E. coli strains containing 07-LPS are found in cases of septicemic infections in neonates (1,36). The 07 antigen is a component of the virulence repertoire in these strains, since it confers resistance to the lytic activity mediated by serum complement (1). We recently reported the cloning and expression in E. coli K-12 of the 07-LPS region from the strain VW187, an E. coli 07:K1 clinical isolate (35). Our data showed that the amount of 07-LPS expressed in E. coli K-12 by 07+ recombinant plasmids was considerably lower than that produced by the wild-type strain, VW187. In this article, we describe the genetic characterization by transposition mutagenesis of the cloned 07-LPS gene cluster. We also present evidence that the cloned DNA contains the essential information for the expression of the 07 polysaccharide, since 07-deficient mutants of strain VW187 generated by sitedirected mutagenesis can be complemented with the 07+ recombinant plasmids.  genes of VW187 were generated by homologous recombination with subsets of the cloned 07-LPS genes subcloned into the suicide mobilizable vector pGP704 as described by Miller and Mekalanos (23). This plasmid vector cannot replicate unless the replication protein is supplied in trans by a gene cloned into a lambda bacteriophage that is integrated in the bacterial chromosome (16). Thus, upon mobilization from strain SM10 Apir to MV103, a Nalr derivative of VW187, the recombinant plasmids integrated into the chromosome due to the homology between insert DNA and chromosomal sequences. The sites of integration were confirmed by DNA hybridization experiments (see below). Mutant strains were examined by coagglutination, sensitivity to rough bacteriophages, and LPS pattern profiles. Some of these mutants were also complemented with plasmids pJHCV31 and pJHCV32, which were mobilized by triparental matings carried out as previously described (35). LPS analysis. LPS was extracted from cell envelopes by using a modification of current procedures (14,32). Cell envelopes obtained from 20-ml cultures were prepared as described previously (35), suspended in 50 ,ul of a solution containing 10 mM Tris hydrochloride (pH 8.0) and 2% SDS, and boiled for 10 min. Proteinase K was added to a final concentration of 800 ,ug/ml, and the samples were incubated at 60°C for 1 h. Proteins were extracted with an equal volume of 90% phenol at 65°C for 15 min. The aqueous phase remaining after centrifugation was extracted once with 10 volumes of ethyl ether to remove traces of phenol, mixed in a 1:1 ratio with a tracking dye solution (125 mM Tris hydrochloride [pH 6.8], 2% SDS, 20% [vol/vol] glycerol, 0.002% bromophenol blue, 10% mercaptoethanol), and boiled for 5 min before the gels were loaded. Gels (14% polyacrylamide; ratio acrylamide to bisacrylamide, 44:0.8) containing 4 M urea were eletrophoresed at 20 mA constant current (32). Immunoblot analysis was carried out as previously described (31,35 large-scale purifications of plasmid DNA (from 800-ml cultures) were carried out as described by Goto et al. (10). Genomic DNA was obtained by the miniscale procedure of Owen and Borman (25). Electrophoresis of plasmid DNA was performed as previously described (33). Transformations were carried out by the calcium chloride method (7) Fig. 3. DNA fragments were recovered by electrophoresis into a strip of DE-81 filter paper (Whatman, Inc., Clifton, N.J.) as previously described (35) and labeled with [32P]ATP (Amersham Canada Ltd., Oakville, Ontario) by oligonucleotide synthesis (9). Southern blot hybridizations were carried out as described previously (33,36).
Analysis of polypeptide gene products. The expression in vivo of polypeptides encoded by pJHCV64, pCMV10, and pCMV21, high-copy-number recombinant plasmids spanning the 07-LPS region ( and DH5a cells with no plasmids were used as controls. Also, purified plasmid DNAs were used as templates for in vitro transcription-translation with a commercial kit available from Amersham. For these experiments, [35S] methionine was used as recommended by the supplier. Polypeptides were separated by SDS-polyacrylamide gel electrophoresis followed by treatment with En3Hance (Dupont, NEN Research Products, Boston, Mass.). Dried gels were exposed to a Kodak X-Omat film at -110°C for 24 to 48 h.

RESULTS
Analysis of the insertion mutatios in the 07-LPS region of pJHCV31 and pJHCV32. Tn3HoHol transposon insertions in plasmids pJHCV31 and pJHCV32 were obtained and mapped as described in Materials and Methods. The mappihg strategy not only localized the insertions to a given restriction endonuclease fragment but also made it possible to establish the orientation of the transposon insertion with respect to the target DNA sequences. A map of transposon insertions resulting from these experiments is shown in Fig. 1.
The distribution of Tn3HoHol insertions along the target DNA was not completely at random, a result expected from a transposon derived from the Tn3 family (12). Most of the insertions in pJHCV32 were located within a region of about 12 kilobase pairs (kb) between HindIII fragments G and H and in fragments C and F (Fig. 1). HindIII fragments B and D did not receive any insertion, although over 300 insertion mutants were screened. This suggests that either the insertions in these regions determine a phenotype that is lethal for bacterial cells or, alternatively, transposition events within these DNA sequences occur at a very low frequency. In the case of pJHCV31, most of the insertions were located in fragments J, A, and H (Fig. 1). Approximately 30% of the insertions in pJHCV31 gave rise to deleted plasmids lacking fragments J, D, and G or only J and D (data not shown). The deletion plasmids were not further studied. pJHCV32::Tn3HoHol-7L pJHCV32::Tn3HoHol-11L pJHCV32: :Tn3HoHol-3R pJHCV32::Tn3HoHol-1OR pJHCV32::Tn3HoHol-8R pJHCV32::Tn3HoHol-lL pJHCV32::Tn3HoHol-2L pJHCV32: :Tn3HoHol-29L pJHCV32: :Tn3HoHol-24R pJHCV32: :Tn3HoHol-140R pJHCV32::Tn3HoHol-40L pJHCV32::Tn3HoHol-23R pJHCV32::Tn3HoHol-50R pJHCV32::Tn3HoHol-26R pJHCV32::Tn3HoHol-27R pJHCV32::Tn3HoHol-47L pJHCV32::Tn3HoHol-58R pJHCV32::Tn3HoHol-21R pJHCV32::Tn3HoHol-136L pJHCV32::Tn3HoHol-20L pJHCV32::Tn3HoHol-31L pJHCV32::Tn3HoHol-37L pJHCV32: :Tn3HoHol-159L pJHCV32: : Wild type  I  I  III  I  I  I  I  I  I  I  I  I  I  I  Wild type  III  III  II  I  I  I  I  I  I  I  I  I  I  I  II  I  I  I  I  I  I  I  I  II  I  I  I  Wild a The number at the end of the plasmid designation has been used in Fig. 1 to identify the insertions. L indicates that the left end of Tn3HoHol (containing the lac genes) is located to the left end of the map as shown in Fig. 1. R indicate that the left end of the transposon is located to the right end of the map. b Letters refer to the HindIlI fragments of pJHCV31 and pJHCV32 as shown in Fig. 1. c +, Positive result comparable to that of DH5a cells containing either pJHCV31 or pJHCV32; -, negative coagglutination; +, weakly positive coagglutination. d 07+ phenotypes of DH5a cells containing each of the plasmids with Tn3HoHol insertions were assigned as described in the text and in Fig. 2 based on the result of silver-stained polyacrylamide gels and immunoblots. e Production of 0-galactosidase was determined by the lysis method of Putnam and Koch (27) and units calculated according to Miller (22). Values equal or lower than 30 were considered background activity. ND, Not done. f (+) and (-) indicate the lac phenotype mediated by the transposon insertions as determined by plating onto MacConkey agar plates.
g Insertion 134 determined a class II mutant in the E. coli C2110 cell background.
The phenotypes encoded by the mutated plasmids in DH5a cells were initially investigated by coagglutination ( Table 2). Insertions conferring a negative coagglutination were located in HindIll fragments J, D, E, A, and H. One insertion in fragment J, two insertions in fragment G, one insertion in fragment E, and two insertions in fragment A determined a weak positive coagglutination. One insertion in fragment J, one insertion in fragment H, and all of the insertions in fragments C and F gave positive coagglutinations comparable with those obtained with DH5a cells containing either pJHCV31 or pJHCV32. These experiments demonstrated that a 2.5-kb portion of fragment J contiguous to fragment D (Fig. 1) is needed for LPS biosynthesis. Therefore, the 07-LPS region spans a contiguous DNA 2. Immunoblot analysis of DH5a cells containing pJHCV31 and/or pJHCV32 derivatives with insertion mutants. Cell envelope fractions extracted with hot phenol as described in Materials and Methods were separated by SDS-polyacrylamide gel electrophoresis and then electrotransferred to nitrocellulose membranes (35). 07specific LPS bands were detected with 07 rabbit polyclonal antiserum as described (35). Lanes: A and H, wild-type 07-LPS determined by pJHCV31 and pJHCV31::Tn3HoHol-406R, respectively; B and C, class III mutants determined by pJHCV31::Tn3HoHol-325R and pJHCV32::Tn3HoHol-128L, respectively; D and F, class II mutants determined by pJHCV32::Tn3HoHol-140R and pJHCV32::Tn3HoHo1-136L, respectively; E and G, class I mutants determined by pJHCV31::Tn3HoHol-306R and pJHCV32::Tn3 HoHol-31L, respectively; I, class II mutant determined by pJHCV32::Tn3HoHol-134L in strain C2110; J, 07-LPS determined by pJHCV32::Tn3HoHol-134 in DH5a. The assignment of classes of mutants is described in the text. Small arrows in lanes B and C denote the 07-specific LPS bands, which are barely visible. Prominent bands in lane C correspond to the presence of residual outer membrane proteins in this preparation that cross-react with the 07 antiserum. CORE indicates the position in the gel of lipid A core molecules, which do not react with the 07-specific antiserum. The coagglutination results are indicated as follows: ++, positive; +, weakly positive; and -, no coagglutination. segment of about 14 kb comprising HindIII fragments J, D, G, E, A, and H (Fig. 1). The limits of this region are defined by insertions 403 and 406, occurring in fragments J and H, respectively (Fig. 1).
Further characterization of the insertion mutants within this region was achieved by silver-stained SDS-polyacrylamide gel electrophoresis and immunoblotting experiments ( Fig. 2 and Table 2). Phenotypically, three classes of 07-LPS mutants were identified: (i) class I mutants, with no 07specific LPS; (ii) class II mutants, showing a single 07reacting LPS band with a molecular weight comparable to that of the lipid A core plus one 07 polysaccharide subunit in the wild type; and (iii) class III mutants, expressing multiple 07-specific LPS bands in very low amounts. Insertions in HindlIl fragments C and F as well as insertion 403 in fragment J and insertion 406 in fragment H encoded an LPS banding pattern identical to that mediated by the control plasmids pJHCV31 and pJHCV32 (Fig. 2, lane H). However, the insertion 134 located in HindIll fragment C determined a class II mutant when pJHCV32: :Tn3HoHo1-134 was present in strain C2110, a polA derivative of E. coli C used for the initial selection of the transposon mutants (Fig. 2, lane I) (35). In contrast, this insertion determined an LPS banding pattern comparable to that of pJHCV32 when examined in the E. coli K-12 strain DH5a cell background (Fig. 2, lane J). Insertion 134 was the only insertion mutant which differed in the phenotype expressed depending on the cell background. This suggests that a function provided by the E. coli K-12 host restores the defect created by the insertion mutant. No information was gained on the participation in the 07-LPS expression of sequences from the HindIII fragment B of pJHCV32, since no transposon insertions in this fragment were found. A site-directed mutagenesis strategy was utilized to establish whether this region plays any role in 07-LPS expression (see below).
Since Tn3HoHol carries a promoterless lactose operon located very close to one of the transposon ends, it can generate gene fusions under the control of promoter sequences located upstream from the insertion (29). In these cases, production of ,-galactosidase serves as an indicator of the direction of transcription. Tn3HoHo1 can insert into sequences in either of two orientations with regard to its lac coding sequences. The letter L was assigned to all of the insertions with the left end of Tn3HoHo1 (containing the lac genes) located to the left end of the map shown in Fig. 1. Conversely, the letter R was given to insertions where the left end of the transposon is located to the right end of the map in Fig. 1. All of the Tn3HoHol insertions in pJHCV31 and pJHCV32 were screened for production of 3-galactosidase. The results demonstrated that the insertions giving a significant amount of P-galactosidase were all located in the L orientation with respect to target DNA sequences in both pJHCV31 and pJHCV32 ( Fig. 1 and Table 2). These insertions expressed ,B-galactosidase levels that were 3to 100fold higher than the background activity of the enzyme, indicating that transcription occurs from fragments J through H. Furthermore, insertions in pJHCV31 expressed approximately 5 to 6 times more 3-galactosidase than those in pJHCV32. It has been suggested by a number of investigators (21) that mutations in glycosyltransferases can compromise the integrity of the bacterial outer membrane and therefore these mutants show an increased sensitivity to dyes and detergents. Transposon insertions in plasmids pJHCV31 and pJHCV32 did not determine any significant changes in the sensitivity of DH5aL cells to sodium deoxycholate and SDS as compared with controls containing either pJHCV31 and pJHCV32 or just DH5a cells with no plasmid (data not shown).
Site-directed mutagenesis in the wild-type strain VW187.
We previously reported that the 07-LPS expression mediated by pJHCV31 and pJHCV32 in E. coli K-12 cells is very low when compared with wild-type levels of 07-LPS produced by the strain VW187 (35). Thus, to determine whether the genetic region defined by transposition mutagenesis in the cloned DNA correlates with the wild-type DNA chromosomal sequences, we constructed 07-deficient derivatives of the wild-type strain VW187. Site-directed mutations were introduced in the wild-type 07-LPS region by using the suicide plasmid vector pGP704 as described in Materials and Methods. Restriction endonuclease fragments from pJHCV31 or pJHCV32 (Fig. 3) were cloned in pGP704 and mobilized by conjugation to MV103, a nalidixic acid-resistant derivative of VW187. Since pGP704 and its derivatives cannot replicate unless the replication protein is provided in trans (23), these experiments resulted in the stable integration of the recombinant plasmids (vector and insert DNA) by homologous recombination between the cloned sequences and chromosomal genes. As a result of the integration the cloned region is then duplicated (23 (35). DNA inserts of the plasmids beneath the VW187 map were used as 32P-radiolabeled probes to confirm the location of the insertions in the chromosomes of the respective mutant strains. obtained from mutant strains and analyzed by agarose gel electrophoresis did not show any changes in plasmid pattern profiles as compared with that of strain VW187. Also, transformation experiments into the strain SY327 A pir with the same plasmid preparations did not yield any Apr colonies, whereas pGP704 transformed this strain efficiently.
Therefore, we concluded that the recombinant suicide plasmids were, indeed, integrated in the chromosome of strain VW187. The correct location of the plasmid insertions into the chromosomal DNA was verified by hybridization experiments with genomic DNA digested with restriction enzymes cleaving outside the regions spanned by the various probe DNAs (Fig. 3). These experiments demonstrated the absence of the wild-type DNA fragment in the region with the integrated plasmid (data not shown). Plasmid insertions 1211, 126 and 122 did not affect the expression of 07-LPS as determined by silver staining and immunoblots with 07specific antibodies (Fig. 4, lanes B, G, and I, respectively). Also these strains were resistant to the lysis by roughspecific bacteriophages (Table 3). Plasmid insertion 1211 was located very close to the site of the transposon insertion 403 in pJHCV31 ( Fig. 1 and 3). Plasmid insertions 126 and 122 were located in HindIII fragments B and C, respectively. These results are consistent with those obtained with trans-poson insertions in either pJHCV31 or pJHCV32 which do not cause any detectable change in the 07-LPS expression ( Fig. 1 and Table 2), and therefore we conclude that these regions are likely not involved in 07-LPS biosynthesis. In contrast, plasmid insertions 129, 124, 123, and 121 lay on regions that are important for 07-LPS biosynthesis (Fig. 3) and, consequently, were sensitive to the lysis by roughspecific bacteriophages (Table 3). 07-deficient mutants of classes I and III were also found in these cases (Fig. 4, lanes  C, D, E, and F). The results are then in agreement with the boundaries of the 07-LPS region defined by transposition mutagenesis in the cloned DNA (Fig. 1). However, the insertion of plasmid pMAV008 (insertion 128) located within HindIII fragment B caused an 07-deficient class I mutant phenotype (Fig. 3, Fig. 4, lanes H; Table 3). Thus, although Tn3HoHol transposon insertions in fragment B of pJHCV32 were not found (Fig. 1), the results obtained in VW187 with the plasmid insertion 128 demonstrate that at least part of the sequences from this fragment encode essential functions for 07-LPS biosynthesis.
The construction of 07-deficient mutants also served to demonstrate whether plasmids pJHCV31 and pJHCV32 can restore the wild-type 07+ phenotype. For this purpose these two plasmids were mobilized by triparental matings into the a Suicide plasmids of the pMAV series are integrated by homologous recombination at various places in the chromosome of strain VW187 (see the text and Fig. 3 for details). b +, Positive; ±, weakly positive; -, negative.
c Sensitivity to rough-specific bacteriophages was tested as previously described (35); S, sensitive to lysis; R, resistant to lysis. 07-deficient strains MV121 and MV124 containing insertions of plasmids pMAV001 and pMAV004, respectively (Fig 3;  Fig. 4 specific bacteriophages (Fig. 5, lanes A through D; data not shown). Thus, it was demonstrated that both pJHCV31 and pJHCV32 must encode essential information for the 07 polysaccharide biosynthesis.
Polypeptide analysis. Since the 14-kb region defined by transposition mutagenesis in pJHCV31 and pJHCV32 contains genetic information for 07-LPS biosynthesis, experiments were attempted to determine the number and species of polypeptide gene products encoded by this region. For this purpose we utilized the high-copy-number plasmid pJHCV64, a subclone of pJHCV32 that contains a 14-kb BstEII fragment from which 12.3 kb correspond to the 07-LPS region (35). pJHCV64 confers a weak but consistent expression of 07-LPS and encompasses most of the 07-LPS region delineated by transposition mutagenesis except for the sequences contained in fragment J (Fig. 3) (35). To examine the contribution of these sequences, plasmids pCMV10 and pCMV21 were also utilized. pCMV10 contains the entire 8.1-kb HindlIl fragment J, whereas pCMV21 contains a 8.5-kb ClaI fragment comprising part of the HindIII fragment J as well as sequences from fragments D, G, E, and A that are common to pJHCV31 and pJHCV32 ( Fig. 1 and 3). Radiolabeled polypeptides encoded by pJHCV64, pCMV10, and pCMV21 were detected by the use of maxicells and also by in vitro transcription-translation. By the maxicell method it was apparent that pJHCV64 encoded a number of polypeptides with molecular masses ranging from 20 to 48 kiodaltons (kDa). However, plasmid-encoded polypeptides were very difficult to resolve in the gels due to the presence of background proteins of chromosomal origin that complicated the interpretation of the results (data not shown). In vitro transcription-translation gave more clearcut results (Fig. 6). pJHCV64 encoded 13 polypeptides with molecular masses of 48, 37, 36.5, 33, 31.5, 29, 28.5 FIG. 6. Autoradiography of polypeptides expressed in an in vitro transcription-translation system. DNA templates were processed as described in Materials and Methods, and polypeptides were labeled with [35S]methionine. The following DNA templates were used (lanes): A, pHP45Q (cloning vector control for pJHCV64); B, pJHCV64; C, pCMV10; D, pCMV21; E, pACYC184 (cloning vector control for pCMV10 and pCMV21); F, pAT153 (internal control for the transcription-translation system); G, 14-kb BstEII fragment from pJHCV64. Lanes A through F were obtained from the same gel, whereas lane G was added from a separate experiment. Dots indicate the polypeptides expressed only by the DNA inserts. Arrows in lane C point at common polypeptides encoded by pCMV10 and pCMV21. Positions of 14C-labeled molecular mass markers (M): bovine serum albumin, 69 kDa; ovalbumin, 46 kDa; carbonic anhydrase, 30 kDa; lysozyme, 14 kDa. (Fig. 6, lane G). In the case of the experiments with pCMV21 DNA, nine polypeptides of 41. 5, 37, 36.5, 33, 31.8, 31.5, 23, 22.8, and 21.5 kDa were detected (Fig. 6, lane D). Six of these polypeptides were also encoded by pJHCV64, whereas polypeptides of 41.5, 31.8, and 22.8 kDa were unique to pCMV21. Also, the polypeptides of 41.5, 31.8, and 22.8 kDa were expressed by pCMV10 (Fig. 6, lane C, arrows). We therefore conclude that these three polypeptides must be encoded by the 2.8-kb DNA ClaI-HindIII segment from the fragment J shared by pCMV10 and pCMV21 and included within the 07-LPS region (Fig. 3).
The sum of the sizes of the 13 polypeptides expressed by pJHCV64 and the three proteins unique to pCMV21 accounts for a coding region of about 14 kb of DNA, assuming that there are no overlaps and noncoding regions between structural genes. This is in good agreement with the molecular size of the 07-LPS region.

DISCUSSION
We report in this work the genetic characterization of the 07-LPS biosynthesis genes from a strain of E. coli 07:K1. Transposon insertions in the cosmids pJHCV31 and pJHCV32 identified a region of approximately 14 kb that is necessary for the expression of an 07+ phenotype in E. coli K-12 cells. Insertions located within the 14-kb region expressed three distinct 07-deficient phenotypes based on the analysis of immunoblots and polyacrylamide gels stained with silver. The most common 07phenotype was that of class I mutants that do not express 07-reacting material in immunoblots and do not show 0-side-chain bands in silverstained gels. Class I mutants denote the existence of genes most likely involved in early steps of the biosynthesis of the 07 oligosaccharide unit. Class II mutants, which express a single side-chain unit linked to lipid A core molecules, indicate lesions of genes involved in the polymerization of the 07 oligosaccharide subunit. This suggests that the gene(s) required for polymerization of the 07 subunit are located within the 07 rIb cluster. Alternatively, it is also possible that some of these mutants are affected in late steps of the biosynthesis of the 07 subunit, resulting in the formation of incomplete single oligosaccharide subunits that cannot be polymerized into a full-length side chain but are ligated to the lipid A core as demonstrated by Sturm et al. (30) in the case of Shigella dysenteriae type I 0-side-chain genes. Chemical analysis will be required to distinguish among the two possibilities. Finally, class III mutants produce a reduced amount of 07-LPS, which may be due to mutations in biosynthesis genes resulting in a decreased enzyme activity or to mutations in regulatory components.
In the case of the cloned E. coli 09 rfb genes, Kido et al. (15) have shown that a partial deletion of the cloned genes results in the synthesis of a new 0-antigenic lipopolysaccharide. We did not find anything similar, since in all cases the results of silver staining and immunoblots gave comparable results.
The fact that one insertion mutant located outside of the 14-kb region mediates different phenotypes depending on the host cell background suggests that gene function(s) present in E. coli K-12 may contribute to the expression of 07-LPS.
In the case of the cloned rfb region from S. dysenteriae type I, the E. coli K-12 chromosome provides some rfb functions that are able to substitute for analogous S. dysenteriae functions (30). The same appears to occur in the cloned 04 and 0101 rJb regions (11,13).
Also, our finding that sequences from the HindIII fragment J unique to pJHCV31 participate in the 07-LPS expression suggests that E. coli K-12 functions analogous to those provided by fragment J contribute to the expression of 07-LPS mediated by pJHCV32. However, these functions are likely not to be encoded by the rib region from E. coli K-12, since pJHCV31 and pJHCV32 can express 07-LPS in strain S0874, which harbors a deletion that eliminates the K-12 rfb region (35). We have also suggested that in E. coli K-12 there is a partial defect in the transfer of 07 polysaccharide chains to the lipid A core (35). Enzyme function(s) provided by the core oligosaccharide biosynthesis genes (rfa gene cluster) are needed for a proper translocation of 0 side chains onto the lipid A core (21). Five core chemotypes are known to occur in E. coli strains, among which the K-12 chemotype appears to be unique (21). It is then possible that translocation enzymes with different specificities may exist, depending on the chemical structure of the core oligosaccharide. This is supported by experiments carried out in the E. coli 08-:K27strain F470, which has an rib mutation and expresses the Rl core chemotype (28). F470 containing pJHCV31 forms 07-LPS at levels comparable to those in the wild-type strain VW187 and becomes resistant to roughspecific bacteriophages (M. A. Valvano, unpublished observations).
When inserted in the proper orientation downstream from an indigenous promoter, Tn3HoHol can generate transcriptional fusions with lac operon sequences (29). Transposon insertions can therefore be screened for the production of P-galactosidase, and the direction of transcription of target genes can be predicted. Our results indicate that transcription occurs in only one direction along the 14-kb region VOL. 172, 1990 containing the 07-LPS genes. However, it is likely that the 07-LPS region consists of various genetic units. This is in part suggested by trans-complementation experiments in E. do not express ,-galactosidase despite being inserted in the proper orientation, suggesting that these insertions may be located in the proximity of promoter regions. All of these observations support the idea that the 07-LPS region is organized as a gene cluster rather than a single operon.
The differential expression of 07-LPS between pJHCV31 and pJHCV32 (35) and the overall low expression by the two plasmids in E. coli K-12 prompted us to confirm the results of transposon mutagenesis by constructing site-directed mutants in the wild-type strain VW187. Suicide plasmids carrying subsets of the 07-LPS region became integrated into the chromosomal DNA by homologous recombination, thereby generating in some cases 07-deficient mutants from the wild-type strain VW187. The location of the plasmid insertions and the resulting phenotypes of the mutant strains parallel the results obtained by transposon mutagenesis in pJHCV31 and pJHCV32. These two plasmids complemented the 07-deficient mutants, confirming that they must contain essential sequences necessary for the 07-LPS biosynthesis.
Furthermore, a locus located outside the 14-kb region not identified by transposition mutagenesis in the cloned DNA was found by this method. A mutation in this region determines a phenotype comparable with that of a class I mutant, suggesting an early blockage of the 07-LPS biosynthesis.
At least 16 proteins with molecular masses ranging from 20 to 48 kDa are encoded by the 14-kb 07-LPS region, as determined by in vitro transcription-translation experiments. This finding is in agreement with the DNA-coding capacity of the 07-LPS region and suggests that the polypeptide genes do not overlap. However, since [35S]methionine was used, it is possible that other proteins lacking methionine or produced in very low amounts are encoded by the 07 region and remained undetected. From studies carried out with S. typhimurium, the rfb gene cluster contains 11 known genes and possibly some other additional, not yet identified genes (21). These genes are involved in the biosynthesis of a tetrasaccharide repeating unit containing the sugars abequose, mannose, rhamnose, and galactose (21). Also in S. typhimurium, rhamnose and mannose alone require five biosynthesis genes located in the rfb region (21). The 07 oligosaccharide repeating subunit consists of a pentasaccharide composed of N-acetylglucosamine, 4-acetamidodideoxyglucose, mannose, rhamnose, and galactose (20). It is then likely that the 16 proteins encoded by the 07-LPS region may be sufficient for the biosynthesis of the five 07-specific sugars and their transfer to the pentasaccharide chain. However, additional functions important for the overall expression of 07-LPS in E. coli K-12 may be located in chromosomal regions other than the 07-LPS region of strain VW187 (35).
In summary, our results demonstrate that the 07-LPS region is genetically complex and consists of a cluster of genes. Construction of a detailed genetic map of polypeptide genes, localization of promoters, and functional studies are under way in our laboratory to elucidate the fine molecular organization of the 07-LPS biosynthesis genes.