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Journal of Bacteriology, March 2007, p. 2571-2574, Vol. 189, No. 6
0021-9193/07/$08.00+0 doi:10.1128/JB.01761-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Department of Microbiology and Immunology, Loyola University Chicago, 2160 South First Ave., Bldg. 105, Maywood, Illinois 60153
Received 17 November 2006/ Accepted 2 January 2007
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FIG. 1. The PTS and the proposed Mif pathways. CM, cytoplasmic membrane; l-di-GMP, linear di-GMP; EIIC and EIIB, subunits of the glucose-specific PTS transporter; EIIA, HPr, and EI, signaling components of the PTS pathway; Pyr, pyruvate. Dotted lines represent a pathway known to function in E. coli. See the text for the proposed interaction between the PTS and Mif pathways (a through d) and their effects upon flagellar biogenesis and on bioluminescence.
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Insights into the control of c-di-GMP production and its targets have come from our investigations of motility in the marine bacterium Vibrio fischeri. This bacterium, found as free-living, motile individuals or as a sessile community in association with the Hawaiian squid Euprymna scolopes, regulates the biogenesis of its flagella in response to the concentration of environmental magnesium (Mg2+). In the presence of abundant Mg2+, such as that found in seawater, V. fischeri cells possess flagella; when Mg2+ becomes limiting, they do not (14). This Mg2+-dependent induction of flagellar (Mif) biogenesis depends upon at least two DGCs, one confirmed (MifA) and one putative (MifB) (Fig. 1). Unlike wild-type (WT) cells, mutants that lack MifA or MifB or both synthesize flagellin proteins and become motile even when Mg2+ is limiting (15). Since MifA (and likely also MifB) catalyzes the production of c-di-GMP, these observations are consistent with a model in which c-di-GMP inhibits flagellar biogenesis, while Mg2+ interferes with or overcomes that inhibition. Since overexpression of mifA did not exert a significant effect upon the transcription of flagellin genes, we have proposed that c-di-GMP acts posttranscriptionally (15).
Disruption of both mifA and mifB under Mg2+-limiting conditions failed to promote the same level of migration that WT cells achieve in the presence of Mg2+; this result suggests the existence of additional Mif pathway components. Therefore, during a recent study (15), we searched for those additional components using a screen for transposon (Tn) mutants that migrated in the absence of Mg2+. The majority of the putative mutations mapped to mifA; however, several did not. Here, we report the characterization of one of the non-mifA mutants.
This non-mifA mutant (strain KV2657) exhibited a phenotype similar to those of mifA and mifB mutants: in the absence of Mg2+, like the mifA vector integration (Campbell) mutant (mifA::pKV217; strain KV2672) or the mifB Campbell mutant (mifB::pTMO125; strain KV2532), it migrated sooner and more rapidly than the positive control, KV1421 (Fig. 2A and B). In the presence of Mg2+, the migration rate of KV2657, like that of mutant mifA or mifB, was similar to that of the KV1421 control (Fig. 2A and C). Furthermore, even with Mg2+, the control strain left a dense mass of poorly motile and nonmotile cells at the site of inoculation. The Tn mutant, in contrast, did not, consistent with the hypothesis that a larger proportion of its cells became motile sooner than those of the control (Fig. 2A and data not shown), a behavior reminiscent of mif mutants (15).
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FIG. 2. Disruption of the ptsHI-crr operon permits migration in the absence of added Mg2+. (A) Cells were grown overnight at 28°C in tryptone broth-saline (TBS) (5), inoculated onto the surface of TBS motility plates containing either no added Mg2+ (Mg) or containing 35 mM MgSO4 (TBS-Mg2+) (+Mg) (14), incubated at 28°C for 5 hours, and photographed. wt refers to strain KV1421 and ptsI refers to strain KV2801 (ptsI::pTMO151), while crr refers to strain KV2850 (crr::pTMO152). (B and C) Displacement of the outer band of strains: KV1421 (WT; solid squares), KV2672 (mifA::pKV217; open squares), KV2532 (mifB::pTMO125; open diamonds), KV2657 (ptsI::Tn5; closed circles), KV2801 (ptsI::pTMO151; open circles), and KV2850 (crr::pTMO152; open triangles) in TBS (B) or in TBS-Mg2+ (C).
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FIG. 3. Arrangement of PTS genes. (A) Schematic shows the putative operon structure, the gene products, and the approximate locations of the Tn5 (inverted triangle) and the primers (inverted arrows) used for both the Campbell insertions and the RT-PCR shown in panel B. Bent arrows indicate putative promoters. (B) RT-PCR, performed as described previously (15), shows that the Campbell insertion allele ptsI::pTMO151 (strain KV2801) is not polar on the downstream crr gene. The parent strain KV1421 is shown as a control. ptsI, primers specific for ptsI; crr, primers specific for crr. Lanes 1, 2, 5, 6, strain KV1421; lanes 3, 4, 7, 8, strain KV2801; lane 9, negative control (distilled water); lane 10, positive control (ESR1 genomic DNA).
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To confirm the role of ptsI in Mg2+-dependent motility, we compared the migration behavior of strain KV2801 (the Campbell mutant) to that of strain KV2657, the original ptsI Tn insertion mutant (Fig. 2B). The Campbell mutant exhibited a similar behavior: in the absence of Mg2+, it migrated sooner and faster than the control strain KV1421 (Fig. 2A and B); in the presence of Mg2+, it behaved much like KV1421. Disruption of ptsI should preclude the phosphorylation of downstream components of the PTS pathways, including those specific for glucose, e.g., EIIAGlc (Fig. 1). To determine whether EIIAGlc also plays a role in controlling motility, we tested the crr Campbell mutant (strain KV2850): in the absence of Mg2+, it began migrating considerably sooner than its WT parent (Fig. 2A and B). In the presence of Mg2+, however, the crr mutant migrated quite a bit more slowly than either the WT or the mifA mutant (Fig. 2A and C). The reason for this behavior remains unknown but is unlikely to be due to differences in growth rate (data not shown).
The behavior of the ptsI mutant could be due either to an inability to phosphorylate downstream components or to a polar effect on the transcription of the downstream crr gene. In E. coli, the ptsHI-crr locus contains two minor promoters positioned upstream of crr within ptsI (10, 17, 26). Thus, a polar mutation in the E. coli ptsI gene does not eliminate transcription from crr (17). To determine whether this is also true of the V. fischeri ptsHI-crr locus, we performed reverse transcription (RT)-PCR (15) using primer pairs specific for ptsI and crr (Fig. 3A). In contrast to the control cells (strain KV1421), which possessed transcripts for both ptsI and crr, the Tn insertion mutant (strain KV2657) showed a transcript for only crr (Fig. 3B). These data demonstrate that the Tn insertion into ptsI does not exert polarity on the downstream crr. Thus, like its E. coli ortholog, the V. fischeri crr gene likely possesses its own promoter, and the effect of a ptsI deficiency likely results from an inability to phosphorylate EIIAGlc and/or other downstream components.
In E. coli, phospho-EIIAGlc activates flagellar biogenesis by enhancing AC activity. The resultant cAMP binds CRP, which activates transcription of flhDC, the operon that encodes the master activator of the E. coli flagellar regulon (11, 28). In contrast, the data for V. fischeri (which uses a regulatory system that does not include flhDC) are consistent with a negative role for phospho-EIIAGlc, much like that for MifA, which appears to act subsequent to transcription (15). To determine whether EI and/or EIIAGlc also act posttranscriptionally, we performed a semiquantitative RT-PCR analysis with flaA, flaC, and flaE, which encode major V. fischeri flagellins (12), and with fliF, predicted to encode the M ring, whose insertion into the cytoplasmic membrane constitutes the first step in building flagella (19). The cDNA levels were not substantially affected by the status of mifB (strain KV2825), ptsI (KV2801), or crr (KV2850) (data not shown). In contrast, the cDNA levels were substantially diminished but not eliminated by the disruption of rpoN (data not shown), which encodes
54, proposed to sit near or at the top of the hierarchical V. fischeri flagellar regulon (strain KV1513) (19, 29). We conclude that both the PTS and the Mif pathway impact flagellar biogenesis at similar levels.
Thus, we propose that phospho-EIIAGlc interacts with the Mif system to regulate flagellar biogenesis, most likely at a posttranscriptional level (Fig. 1). This model is supported by the following observations: (i) ptsI and crr mutants of V. fischeri exhibited similar behavior, i.e., increased migration in the absence of Mg2+ (Fig. 2), suggesting that phospho-EIIAGlc is a prerequisite for the inhibition of flagellar biogenesis; (ii) disruption of either ptsI or crr exerted no significant effect upon the transcription of flagellin genes; and (iii) these behaviors resemble those of mif mutants (15). We do not yet know how the Mif and PTS pathways interact, although we can imagine an interaction between phospho-EIIAGlc and the Mif pathway at any one of a number of levels (Fig. 1): phospho-EIIAGlc could (i) activate a DGC; (ii) inhibit a PDE composed of an EAL domain and a nonconsensus "regulatory" GGDEF domain (21); (iii) interfere with the ability of c-di-GMP to interact with its target; or (iv) act independently. EIIAGlc, either in its phosphorylated or nonphosphorylated form, tends to interact with a variety of proteins, including not only EIICBGlc and AC (5a, 17) but also FrsA, which controls the fermentation-respiration switch (9). Alternatively, the effect could be mediated by another PTS component, e.g., EI or HPr.
To further our understanding of the relationship between the PTS and the Mif pathway, we also considered the following published observations: (i) in E. coli, phospho-EIIAGlc activates AC to produce cAMP (6, 17, 20); (ii) cAMP binds and activates the transcriptional regulator CRP (30); and (iii) CRP of V. fischeri has been linked to the control of bioluminescence (J. L. Bose, U. Kim, W. Bartkowski, R. P. Gunsalus, A. M. Overley, K. L. Visick, and E. V. Stabb, submitted for publication; 7, 8, 13). We therefore asked whether a mutation in mifA, mifB, ptsI, or crr impacted bioluminescence. We grew WT and mutant cells in a tryptone-based seawater medium (which contains Mg2+) and measured bioluminescence over time. To account for any small differences in growth, we plotted the specific luminescence (relative light units/ml/optical density unit) versus optical density (Fig. 4). Cells defective for mifA (KV2672) or mifB (KV2532) or both (KV2826) displayed bioluminescence patterns comparable to that of the WT strain (ES114). Surprisingly, however, mutants defective for either ptsI (KV2801) or crr (KV2850) exhibited substantially elevated levels of bioluminescence. Specifically, these mutations promoted an increase of over 300-fold in bioluminescence, allowing this naturally nonvisibly bioluminescent strain of V. fischeri to produce visibly detectable light. Given the clear differences in the bioluminescence patterns of these strains, these data support the conclusion that Mif and the PTS function in distinct pathways, at least with respect to bioluminescence.
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FIG. 4. The ptsHI-crr operon regulates bioluminescence. Cells were grown at 28°C in SWT (3) supplemented with extra NaCl (for a final concentration of 552 mM), and bioluminescence was measured over time. To account for any small differences in growth, specific luminescence (relative light units/ml/optical density unit [OD]) was plotted versus optical density. The experiment was performed with two independent isolates (denoted as solid and dotted lines) of the following strains: ES114 (WT, closed squares), KV2532 (mifB::pTMO125, open diamonds), KV2672 (mifA::pKV217, open squares), KV2801 (ptsI::pTMO151, open circles), KV2850 (crr::pTMO152, open triangles), and KV2826 (mifA::pKV217 mifB, gray squares).
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Published ahead of print on 12 January 2007. ![]()
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54 controls motility, biofilm formation, luminescence, and colonization. Appl. Environ. Microbiol. 70:2520-2524.This article has been cited by other articles:
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