The region immediately downstream from the miaA tRNA modification gene at 94.8 min contains the hfq gene and the hflA region, which are important in the bacteriophage Q beta and lambda life cycles. The roles of these genes in bacteria remain largely unknown. We report here the characterization of two chromosomal hfq insertion mutations. An omega (omega) cassette insertion near the end of hfq resulted in phenotypes only slightly different from the parent, although transcript mapping demonstrated that the insertion was completely polar on hflX expression. In contrast, an equally polar omega cassette insertion near the beginning of hfq caused pronounced pleiotropic phenotypes, including decreased growth rates and yields, decreased negative supercoiling of plasmids in stationary phase, increased cell size, osmosensitivity, increased oxidation of carbon sources, increased sensitivity to ultraviolet light, and suppression of bgl activation by hns mutations. hfq::omega mutant phenotypes were distinct from those caused by omega insertions early in the miaA tRNA modification gene. On the other hand, both hfq insertions interfered with lambda phage plaque formation, probably by means of polarity at the hflA region. Together, these results show that hfq function plays a fundamental role in Escherichia coli physiology and that hfq and the hflA-region are in the amiB-mutL-miaA-hfq-hflX superoperon.

SUMMARY In ellipsoid-shaped ovococcus bacteria, such as the pathogen Streptococcus pneumoniae (pneumococcus), side-wall (peripheral) peptidoglycan (PG) synthesis emanates from midcells and is catalyzed by the essential class B penicillin-binding protein PBP2b transpeptidase (TP). We report that mutations that inactivate the pneumococcal YceG-domain protein, Spd_1346 (renamed MltG), remove the requirement for PBP2b. ΔmltG mutants in unencapsulated strains accumulate inactivation mutations of class A PBP1a, which possesses TP and transglycosylase (TG) activities. The “synthetic viable” genetic relationship between Δpbp1a and ΔmltG mutations extends to essential ΔmreCD and ΔrodZ mutations that misregulate peripheral PG synthesis. Remarkably, the single MltG(Y488D) change suppresses the requirement for PBP2b, MreCD, RodZ, and RodA. Structural modeling and comparisons, catalytic-site changes, and an interspecies chimera indicate that pneumococcal MltG is the functional homologue of the recently reported MltG endo-lytic transglycosylase of Escherichia coli. Depletion of pneumococcal MltG or mltG(Y488D) increases sphericity of cells, and MltG localizes with peripheral PG synthesis proteins during division. Finally, growth of Δpbp1a ΔmltG or mltG(Y488D) mutants depends on induction of expression of the WalRK TCS regulon of PG hydrolases. These results fit a model in which MltG releases anchored PG glycan strands synthesized by PBP1a for crosslinking by a PBP2b:RodA complex in peripheral PG synthesis.

Bacterial cell division and peptidoglycan (PG) synthesis are orchestrated by the coordinated dynamic movement of essential protein complexes. Recent studies show that bidirectional treadmilling of FtsZ filaments/bundles is tightly coupled to and limiting for both septal PG synthesis and septum closure in some bacteria, but not in others. Here we report the dynamics of FtsZ movement leading to septal and equatorial ring formation in the ovoid-shaped pathogen, Streptococcus pneumoniae. Conventional and single-molecule total internal reflection fluorescence microscopy (TIRFm) showed that nascent rings of FtsZ and its anchoring and stabilizing proteins FtsA and EzrA move out from mature septal rings coincident with MapZ rings early in cell division. This mode of continuous nascent ring movement contrasts with a failsafe streaming mechanism of FtsZ/FtsA/EzrA observed in a ΔmapZ mutant and another Streptococcus species. This analysis also provides several parameters of FtsZ treadmilling in nascent and mature rings, including treadmilling velocity in wild-type cells and ftsZ(GTPase) mutants, lifetimes of FtsZ subunits in filaments and of entire FtsZ filaments/bundles, and the processivity length of treadmilling of FtsZ filament/bundles. In addition, we delineated the motion of the septal PBP2x transpeptidase and its FtsW glycosyl transferase-binding partner relative to FtsZ treadmilling in S. pneumoniae cells. Five lines of evidence support the conclusion that movement of the bPBP2x:FtsW complex in septa depends on PG synthesis and not on FtsZ treadmilling. Together, these results support a model in which FtsZ dynamics and associations organize and distribute septal PG synthesis, but do not control its rate in S. pneumoniae.

SUMMARY GpsB regulatory protein and StkP protein kinase have been proposed as molecular switches that balance septal and peripheral (side-wall like) peptidoglycan (PG) synthesis in Streptococcus pneumoniae (pneumococcus); yet, mechanisms of this switching remain unknown. We report that ΔdivIVA mutations are not epistatic to ΔgpsB division-protein mutations in progenitor D39 and related genetic backgrounds; nor is GpsB required for StkP localization or FDAA labeling at septal division rings. However, we confirm that reduction of GpsB amount leads to decreased protein phosphorylation by StkP and report that the essentiality of ΔgpsB mutations is suppressed by inactivation of PhpP protein phosphatase, which concomitantly restores protein phosphorylation levels. ΔgpsB mutations are also suppressed by other classes of mutations, including one that eliminates protein phosphorylation and may alter division. Moreover, ΔgpsB mutations are synthetically lethal with Δpbp1a, but not Δpbp2a or Δpbp1b mutations, suggesting GpsB activation of PBP2a activity. Consistent with this result, co-IP experiments showed that GpsB complexes with EzrA, StkP, PBP2a, PBP2b, and MreC in pneumococcal cells. Furthermore, depletion of GpsB prevents PBP2x migration to septal centers. These results support a model in which GpsB negatively regulates peripheral PG synthesis by PBP2b and positively regulates septal ring closure through its interactions with StkP-PBP2x.

The MutS, MutL, and MutH proteins play major roles in several DNA repair pathways. We previously reported that the cellular amounts of MutS and MutH decreased by as much as 10-fold in stationary-phase cultures. Consequently, we tested whether the amounts of MutS, MutL, and MutH were regulated by two global regulators, RpoS ( 38 ) and Hfq (HF-I [putative RNA chaperone]), which are involved in stationary-phase transition. We report here that mutations in hfq and rpoS reversed the stationary-phase down-regulation of the amounts of MutS and MutH. hfq regulation of the amount of MutS in stationary-phase cultures was mediated by RpoS-dependent and -independent mechanisms, whereas hfq regulation of the amount of MutH was mediated only through RpoS. Consistent with this interpretation, the amount of MutS but not MutH was regulated by Hfq, but not RpoS, in exponentially growing cells. The amount of MutL remained unchanged in rpoS, hfq-1, and rpoS ؉ , hfq ؉ strains in exponentially growing and stationary-phase cultures and served as a control. The ␤-galactosidase activities of single-copy mutS-lacZ operon and gene fusions suggested that hfq regulates mutS posttranscriptionally in exponentially growing cultures. RNase T 2 protection assays revealed increased amounts of mutS transcript that are attributed to increased mutS transcript stability in hfq-1 mutants. Lack of Hfq also increased the amounts and stabilities of transcripts initiated from P miaA and P1 hfq HS, two of the promoters for hfq, suggesting autoregulation, but did not change the half-life of bulk mRNA. These results suggest that the amounts of MutS and MutH may be adjusted in cells subjected to different stress conditions by an RpoS-dependent mechanism. In addition, Hfq directly or indirectly regulates several genes, including mutS, hfq, and miaA, by an RpoS-independent mechanism that destabilizes transcripts.The MutS, MutL, and MutH proteins of Escherichia coli play important roles in several DNA repair pathways and the maintenance of chromosomal stability (51, 53). In the methyl-directed-mismatch (MDM) repair pathway, MutS, MutL, and MutH function as a DNA mismatch binding protein (75, 76), a putative molecular matchmaker (66), and a d(GATC)-specific endonuclease (83), respectively. MDM repair is a major pathway that sets spontaneous mutation rates by correcting mismatched base pairs and small single-strand loops that arise as replication errors (9,60,75). In addition, MutL and MutS play roles in very-short-patch repair (38, 40), transcription-coupled nucleotide excision repair (48), and the prevention of homeologous recombination (61, 62). Homologs of E. coli MutL and MutS have been found in most other bacteria and in eukaryotes (26, 51, 53), in which they play crucial roles in DNA repair, in maintaining chromosome stability, in meiotic recombination, and in preventing colon and sporadic cancers (31,52,53).Direct measurements showed that MutS and MutL proteins are present at about 186 and 113 dimers, respectively, per exponentially growing cell (15), wher...

SUMMARY Suppressor mutations were isolated that obviate the requirement for essential PBP2b in peripheral elongation of peptidoglycan from the midcells of dividing Streptococcus pneumoniae D39 background cells. One suppressor was in a gene encoding a single KH-domain protein (KhpA). ΔkhpA suppresses deletions in most, but not all (mltG), genes involved in peripheral PG synthesis and in the gpsB regulatory gene. ΔkhpA mutations reduce growth rate, decrease cell size, minimally affect shape, and induce expression of the WalRK cell-wall stress regulon. Reciprocal co-immunoprecipitations show that KhpA forms a complex in cells with another KH-domain protein (KhpB/JAG/EloR). ΔkhpA and ΔkhpB mutants phenocopy each other exactly, consistent with a direct interaction. RNA-immunoprecipitation showed that KhpA/KhpB bind an overlapping set of RNAs in cells. Phosphorylation of KhpB reported previously does not affect KhpB function in the D39 progenitor background. A chromosome duplication implicated FtsA overproduction in Δpbp2b suppression. We show that cellular FtsA concentration is negatively regulated by KhpA/B at the post-transcriptional level and that FtsA overproduction is necessary and sufficient for suppression of Δpbp2b. However, increased FtsA only partially accounts for the phenotypes of ΔkhpA mutants. Together, these results suggest that multimeric KhpA/B may function as a pleiotropic RNA chaperone controlling pneumococcal cell division.

PcsB is a protein of unknown function that plays a critical role in cell division in Streptococcus pneumoniae and other ovococcus species of Streptococcus. We constructed isogenic sets of mutants expressing different amounts of PcsB in laboratory strain R6 and virulent serotype 2 strain D39 to evaluate its cellular roles. Insertion mutagenesis in parent and pcsB ؉ merodiploid strains indicated that pcsB is essential in serotype 2 S. pneumoniae. Quantitative Western blotting of wild-type and epitope-tagged PcsB showed that all PcsB was processed into cell-associated and secreted forms of the same molecular mass and that cell-associated PcsB was moderately abundant and present at Ϸ4,900 monomers per cell. Controlled expression and complementation experiments indicated that there was a causative relationship between the severity of defects in cell division and decreasing PcsB amount. These experiments also showed that perturbations of expression of the upstream mreCD genes did not contribute to the cell division defects of pcsB mutants and that mreCD could be deleted. Unexpectedly, capsule influenced the cell shape and chain formation phenotypes of the wild-type D39 strain and mutants underexpressing PcsB or deleted for other genes involved in peptidoglycan biosynthesis, such as dacA. Underexpression of PcsB did not result in changes in the amounts or composition of lactoyl-peptides, which were markedly different in the R6 and D39 strains, and there was no correlation between decreased PcsB amount and sensitivity to penicillin. Finally, microarray analyses indicated that underexpression of PcsB may generate a signal that increases expression of the VicRK regulon, which includes pcsB.Comparatively little is known about the exact mechanisms and interactions that occur during peptidoglycan (PG) biosynthesis and cell division of ovococcus bacteria, such as Streptococcus pneumoniae (pneumococcus) and other Streptococcus species (reviewed recently in reference 71). These ovoid, football-shaped bacteria divide along successive parallel planes perpendicular to the long axis of the cell. This mode of division contrasts with that of spherical coccus species such as Staphylococcus aureus, which divides in alternating perpendicular planes. Another major difference between these two bacterial types is that ovococcus species have two modes of PG biosynthesis (peripheral and septal), whereas spherical species have only septal PG biosynthesis (30,38,50,51,71). All models of PG biosynthesis require murein hydrolases, such as amidases or endopeptidases, to participate in PG remodeling and cell separation during division (15,48,66,67,71).One candidate murein hydrolase is the PcsB protein, which has been characterized to various degrees in S. pneumoniae (29,42,45,46), Streptococcus mutans (where it is called GbpB) (11,12,40,41), and Streptococcus agalactiae (55,56). PcsB orthologs contain four domains: a signal peptide (SP) that directs export, a predicted extended coiled-coil domain that contains leucine zipper motifs, an alanine-ric...

Summary spxB-encoded pyruvate oxidase is a major virulence factor of Streptococcus pneumoniae. During aerobic growth, SpxB synthesizes H2O2 and acetyl phosphate, which play roles in metabolism, signalling, and oxidative stress. We report here the first cis-and transacting regulatory elements for spxB transcription. These elements were identified in a genetic screen for spontaneous mutations that caused colonies of strain D39 to change from a semitransparent to an opaque appearance. Six of the seven opaque colonies recovered (frequency ª 3 ¥ 10