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Journal of Bacteriology, November 2007, p. 8371-8376, Vol. 189, No. 22
0021-9193/07/$08.00+0 doi:10.1128/JB.01217-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Avenida da República, Apartado 127, 2781-901 Oeiras, Portugal,1 Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal2
Received 28 July 2007/ Accepted 30 August 2007
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-arabinanases (ABN; EC 3.2.1.99) and
-L-arabinofuranosidases (AF; EC 3.2.1.55), capable of releasing arabinosyl oligomers and arabinose from the plant tissue homoglycan arabinan (1, 13, 14, 35). In previous studies, we characterized the transcriptional regulation of three arabinan-degrading enzyme genes, abnA (ABN), xsa (AF), and abfA (AF), which are clustered with genes encoding enzymes that further catabolize L-arabinose (23). Expression of these genes is (i) induced by arabinose and arabinan, (ii) negatively controlled by AraR, the key regulator of the arabinose regulon, and (iii) repressed in the presence of glucose (6, 12, 23). The last phenomenon, carbon catabolite repression (CCR), is a global regulatory mechanism that ensures the selection of rapidly metabolized carbon and energy sources, such as glucose, for optimal bacterial growth (3, 30). CcpA is the master regulator of CCR in Bacilli and other gram-positive bacteria with low GC contents and regulates approximately 10% of the B. subtilis genome (2, 18). This global regulator binds to cres (catabolite-responsive elements) either upstream of or in the promoter region or within coding regions of target genes (17, 34). This interaction is modulated by phosphorylation of HPr, a phosphocarrier protein of the phosphoenolpyruvate phosphotransferase system (PTS), or of Crh (an HPr-like protein) (4, 30, 33). In this work, we identified trans-acting factors and cis elements involved in glucose repression of arabinan-degrading enzyme genes. Moreover, we address the question of temporal regulation of these genes (23) by determination of the contributions of CcpA and the coeffectors HPr and Crh to glucose repression at different growth stages (exponential and postexponential [transitional] growth).
cis-Regulatory elements in the promoter regions of abnA and xsa.
The involvement of two cres present in the araABDLMNPQ-abfA operon (cre araA and cre araB) (Fig. 1) in CCR was shown in previous studies (12, 17). Both cre araA, located between the promoter region and the araA gene, and cre araB, placed 2 kb downstream within the araB gene, are independently functional, and both contribute to glucose repression (12). Moreover, the level of glucose repression, 21.8-fold, measured in a strain bearing an abfA'-lacZ reporter fusion (IQB450) (23; see below) was only 1.6-fold higher than that observed with a strain bearing an araAB'-lacZ fusion, i.e., 13.4-fold (12), suggesting the absence of other cre active sites in glucose-mediated CCR of abfA expression. Expression of abnA and xsa is also subject to glucose repression (23). Potential cre sequences, namely, cre abnA (+83-TGTAAGCGCTTTCT [SubtiList position 2949135]) (Fig. 1) (17) and cre xsa (+1-TAAAAGCGCTTACA [SubtiList position 2914316]) (Fig. 1) (18), are present in the promoter regions of the genes. To assess the functionality of these putative regulatory sites, we introduced a single-base-pair substitution which destroyed the central symmetry in both cre abnA (+89 C
A) and cre xsa (+7 C
A) (Fig. 1). The two promoter regions, together with the respective mutated cres, were fused to the lacZ reporter gene and introduced into the amyE locus. In the resulting B. subtilis strains, IQB472 and IQB473 (Table 1), respectively, the levels of accumulated ß-galactosidase activity in the absence or presence of the inducer arabinose and under repressing conditions (arabinose plus glucose) were determined. Glucose repression was almost abolished in the mutants compared to that in strains bearing the wild-type promoters (IQB410 and IQB405) (23) (Table 2). These results indicate that both cre abnA and cre xsa are functional and that the designed mutation prevented binding of CcpA (see below) in the presence of glucose. Thus, cre abnA and cre xsa are the cis-acting elements involved in glucose-mediated CCR of abnA and xsa expression at the transcriptional level. In addition, the results suggest that only one cre in each promoter is responsible for this phenomenon.
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FIG. 1. Organization of the B. subtilis chromosome region comprising the arabinan-degrading enzyme genes abnA, abfA, and xsa. The three genes are represented by dotted arrows pointing in the direction of transcription. abfA belongs to the araABDLMNPQ-abfA metabolic operon, abnA is located immediately upstream, and xsa is positioned 23 kb downstream of the metabolic operon. Hairpin structures denote terminators. The dotted boxes below the physical map represent extensions of the inserts fused to the lacZ reporter gene in the indicated plasmids. Plasmid pSA1 was integrated into the host chromosome by means of a single-crossover (Campbell-type) recombinational event that occurred in the region of homology of the resulting strain (Table 1). Linearized DNAs from plasmids pSN40, pZI26, pZI27, and pRIT1 were used to transform B. subtilis strains (Table 1), and the fusions were integrated into the chromosome via double recombination at the amyE locus. Above the map, the DNA sequences of cre abnA and cre xsa are depicted, and the numbers indicate the positions of the abnA and xsa genes relative to the transcriptional start site. The single nucleotide change (C A) introduced into each cre is shown.
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TABLE 1. B. subtilis strains, plasmids, and oligonucleotides used in this study
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TABLE 2. Single-base-pair substitutions in the cres located in the promoter regions of the abnA and xsa genesa
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FIG. 2. Effects of ccpA, ptsH1, crh, hprK, and araR mutations on catabolite repression of the xsa, abnA, and abfA genes. (A) Growth curves for wild-type (squares) and ccpA-null mutant (triangles) B. subtilis strains in C minimal medium (21) supplemented with 1% (wt/vol) casein hydrolysate in the presence of 0.4% (wt/vol) arabinose plus 0.4% (wt/vol) glucose. Exponential growth phase (t2) and transitional phase (t4) are indicated. The strains bearing the xsa'-lacZ (B), abnA'-lacZ (C), and abfA'-lacZ (D) transcriptional fusions were grown in the absence of sugar, in the presence of 0.4% (wt/vol) arabinose, and in the presence of 0.4% (wt/vol) arabinose plus 0.4% (wt/vol) glucose. The levels of accumulated ß-galactosidase activity, expressed as Miller units, were measured at hours 2 (t2) and 4 (t4) after induction (addition of arabinose). The results are represented by the glucose repression index, calculated as the ratio between the level of accumulated Miller units obtained in the presence of arabinose and the value determined in the presence of arabinose plus glucose. Each value is the average of two measurements each from three independent experiments.
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During exponential growth of cells with an araR-null mutant background, we observed a twofold derepression of expression of xsa'-lacZ (IQB406) and abfA'-lacZ (IQB453) in the presence of glucose compared to that of the wild type (Fig. 2B and C). AraR is a negative regulator of the arabinose-inducible genes (6, 7, 20), and this effect was previously observed with araE'-lacZ and araAB'-lacZ fusions (12). The AraR binding sites and cre sequences are located close to each other in the promoter regions of the araABDLMNPQ-abfA, araE, and xsa genes (12, 19, 23). Since under particular physiological conditions both CcpA and AraR might be bound to the DNA in close proximity, a possible interaction (cooperation) between the two proteins in the negative control of these genes was proposed (12, 19). Interestingly, this effect was not observed for the abnA'-lacZ fusion in an araR-null mutant background (strain IQB411) (Fig. 2D; Table 1). This observation is in agreement with a different localization of cre abnA relative to the AraR binding site in the abnA promoter and also to a weak control of abnA expression by AraR as a result of a distinct mechanism of action (23).
Crh expression increases at transitional growth phase. The contribution of both effectors HPr and Crh to CCR by glucose can depend on the growth conditions, as shown for the hut operon, which responds to HPr in rich LB medium and to both HPr and Crh in minimal medium (36). Crh is the major effector for CCR of the citM gene in a medium containing succinate and glutamate (32). In connection with this, it was recently shown that the nature of the carbon source influences ptsH and crh expression (9). Using ptsH'-lacZ and crh'-lacZ transcription and translational fusion analysis, it was suggested that the different contributions of Crh and HPr to CCR might be due to the drastic differences in their synthesis rates under the conditions that cause CCR. In the presence of PTS carbohydrates, such as glucose, the synthesis level of HPr during exponential growth is 100-fold higher than that of Crh, which may explain the minor role of the latter in certain circumstances (9). Moreover, CcpA binds Crh(Ser-P) more weakly than HPr(Ser-P) (28). We have previously shown the absence of a Crh contribution to glucose repression of the araABDLMNPQ-abfA and araE genes in both C minimal medium supplemented with casein hydrolysate and CSK minimal medium (12). To further investigate the lack of Crh dependency during exponential growth and its possible contribution to glucose repression of the arabinan-degrading enzyme genes during the transitional growth phase, we analyzed the expression of both ptsH'-lacZ and crh'-lacZ transcriptional fusions at different growth stages. The constructed transcriptional fusions are identical to those described by Görke et al. (9). In B. subtilis strains bearing the fusions, the complete ptsH and crh coding regions are restored downstream from the lacZ insertion cassette. Their expression is directed by the IPTG (isopropyl-ß-D-thiogalactopyranoside)-inducible Pspac promoter in order to prevent polar effects caused by insertions into their natural chromosomal loci (Table 1). Expression from the crh'-lacZ (IQB495) (Table 1) and ptsH'-lacZ (IQB496) (Table 1) transcriptional fusions during exponential growth phase (t2) was comparable in both C minimal medium supplemented with casein hydrolysate and CSK minimal medium (Table 3). The presence of arabinose, a non-PTS sugar, did not influence crh and ptsH expression (Table 3). In addition, the ß-galactosidase activities obtained for crh'-lacZ and ptsH'-lacZ in CSK minimal medium plus glucose (29 and 229, respectively) (Table 3) were very similar to those obtained by Görke et al. (9). Interestingly, at transitional growth phase (t4), the expression of crh increased about twofold in the presence of glucose and in both media. In contrast, ptsH expression was unaffected by the growth phase. These results suggest that increased expression of crh at the transcriptional level during transitional phase in the presence of glucose may lead to a higher cellular concentration of Crh than of HPr. Thus, this difference in synthesis level may contribute to the glucose repression relief effect on abfA'-lacZ and xsa'-lacZ expression in the crh-null mutant background (Fig. 2). However, due to a still expected higher concentration of HPr than Crh under these conditions, other factors, such as intracellular variation of certain metabolites which may influence Crh(Ser-P) phosphorylation and/or CcpA-Crh(Ser-P) complex formation, are likely to contribute to Crh(Ser-P)-mediated glucose repression at transitional phase.
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TABLE 3. Expression from crh'-lacZ and ptsH'-lacZ fusions in minimal media at different growth stagesa
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This work was partially supported by grants POCTI/AGR/36212/00 and POCI/AGR/60236/2004 from Fundação para a Ciência e Tecnologia (FCT) and FEDER to I.D.S.-N. and by fellowship SFRH/BD/18238/2004 from FCT to J.M.I.
Published ahead of print on 7 September 2007. ![]()
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