Interaction of the Neisseria gonorrhoeae PilA protein with the pilE promoter involves multiple sites on the DNA

Transposon mutagenesis identifies sites upstream of the Neisseria gonorrhoeae pilE gene that modulate pilin antigenic variation

Gene conversion mediates the variation of virulence-associated surface structures on pathogenic microorganisms, which prevents host humoral immune responses from being effective. One of the best-studied gene conversion systems is antigenic variation (Av) of the pilin subunit of the Neisseria gonorrhoeae type IV pilus. To identify cis-acting DNA sequences that facilitate Av, the 700-bp region upstream of the pilin gene pilE was targeted for transposon mutagenesis. Four classes of transposon-associated mutations were isolated, distinguishable by their pilus-associated phenotypes: (i) insertions that did not alter Av or piliation, (ii) insertions that blocked Av, (iii) insertions that interfered with Av, and (iv) insertions that interfered with pilus expression and Av. Mutagenesis of the pilE promoter did not affect the frequency of Av, directly demonstrating that pilin Av is independent of pilE transcription. Two stretches of sequence upstream of pilE were devoid of transposon insertions, and some deletions in these regions were not recoverable, suggesting that they are essential for gonococcal viability. Insertions that blocked pilin Av were located downstream of the RS1 repeat sequence, and deletion of the region surrounding these insertions completely abrogated pilin Av, confirming that specific sequences 5' to pilE are essential for the recombination events underlying pilin Av.

Role for both DNA and RNA in GTP hydrolysis by the Neisseria gonorrhoeae signal recognition particle receptor

The prokaryotic signal recognition particle (SRP) targeting system is a complex of two proteins, FtsY and Ffh, and a 4.5S RNA that targets a subset of proteins to the cytoplasmic membrane cotranslationally. We previously showed that Neisseria gonorrhoeae PilA is the gonococcal FtsY homolog. In this work, we isolated the other two components of the gonococcal SRP, Ffh and 4.5S RNA, and characterized the interactions among the three SRP components by using gel retardation and nitrocellulose filter-binding assays and enzymatic analyses of the two proteins. In the current model of prokaryotic SRP function, based on studies of the Escherichia coli and mammalian systems, Ffh binds to 4.5S RNA and the Ffh-4.5S RNA complex binds to the signal sequence of nascent peptides and then docks with FtsY at the membrane. GTP is hydrolyzed by both proteins synergistically, and the nascent peptide is transferred to the translocon. We present evidence that the in vitro properties of the gonococcal SRP differ from those of previously described systems. GTP hydrolysis by PilA, but not that by Ffh, was stimulated by 4.5S RNA, suggesting a direct interaction between PilA and 4.5S RNA that has not been reported in other systems. This interaction was confirmed by gel retardation analyses in which PilA and Ffh, both alone and together, bound to 4.5S RNA. An additional novel finding was that P(pilE) DNA, previously shown by us to bind PilA in vitro, also stimulates PilA GTP hydrolysis. On the basis of these data, we hypothesize that DNA may play a role in targeting proteins via the SRP.

Neisseria gonorrhoeae PilA is an FtsY homolog

The pilA gene of Neisseria gonorrhoeae was initially identified in a screen for transcriptional regulators of pilE, the expression locus for pilin, the major structural component of gonococcal pili. The predicted protein sequence for PilA has significant homology to two GTPases of the mammalian signal recognition particle (SRP), SRP54 and SRalpha. Homologs of SRP54 and SRalpha were subsequently identified in bacteria (Ffh and FtsY, respectively) and appear to form an SRP-like apparatus in prokaryotes. Of the two proteins, PilA is the most similar to FtsY (47% identical and 67% similar at the amino acid level). Like FtsY, PilA is essential for viability and hydrolyzes GTP. The similarities between PilA and the bacterial FtsY led us to ask whether PilA might function as the gonococcal FtsY. In this work, we show that overproduction of PilA in Escherichia coli leads to an accumulation of pre-beta-lactamase, similar to previous observations with other bacterial SRP components. Low-level expression of pilA in an ftsY conditional mutant can complement the ftsY mutation and restore normal growth to this strain under nonpermissive conditions. In addition, purified PilA can replace FtsY in an in vitro translocation assay using purified E. coli SRP components. A PilA mutant that is severely affected in its GTPase activity cannot replace FtsY in vivo or in vitro. However, overexpression of the GTPase mutant leads to the accumulation of pre-beta-lactamase, suggesting that the mutant protein may interact with the SRP apparatus to affect protein maturation. Taken together, these results show that the gonococcal PilA is an FtsY homolog and that the GTPase activity is necessary for its function.

An AT-rich tract containing an integration host factor-binding domain and two UP-like elements enhances transcription from the pilEp1 promoter of Neisseria gonorrhoeae

The pilE gene of Neisseria gonorrhoeae is transcribed from a sigma70 promoter (pilEp1) with an AT-rich tract extending 65 nucleotides upstream of the -35 box. Within this region is an integration host factor (IHF)-binding core consensus sequence. We have performed a detailed analysis to determine which upstream sequences are required for efficient transcription from pilEp1 in N. gonorrhoeae. Deletion of sequences upstream of the AT-rich tract had no effect on the level of transcription. However, the IHF-binding core consensus sequence and the AT-rich sequence further upstream were both required for enhanced levels of transcription from this promoter in both N. gonorrhoeae and an Escherichia coli strain producing IHF. In addition, an UP-like element positioned between the -35 box and the IHF-binding site was required for maximal transcription. The AT-rich region upstream of the IHF-binding core consensus sequence can also act as an UP-like element when appropriately repositioned upstream of the -35 box.

Isolation and biochemical characterization of the PilA protein of Neisseria meningitidis

PilA is the response regulator of a two-component regulatory system that controls a number of genes in the pathogenic Neisseria. Previous work has shown that Neisseria gonorrhoeae (GC) PilA binds DNA and also hydrolyzes GTP. Here, we report the cloning, sequencing, purification, and biochemical characterization of PilA from N. meningitidis (MC) strain 8013. MC pilA is 94% identical to GC pilA at the nucleotide level. Of the 78 nucleotide changes, 52 are silent, while 26 result in a total of 20 amino acid changes. Additionally, the MC homolog has a 4-amino acid insertion between the putative DNA-binding and GTP-binding domains. Purified MC PilA binds to the same DNA fragment we have previously shown to be bound by GC PilA specifically and also hydrolyzes GTP. The K(m) of MC PilA for GTP is 8.6 microM, similar to that determined for the GC protein. However, the maximum velocity (Vmax) is approximately 35-fold greater than the GC PilA activity. Additionally, the nucleotide specificity of MC PilA differs from that of GC PilA. While GC PilA hydrolyzes only GTP, MC PilA hydrolyzes GTP and ATP equally well, and CTP and UTP also compete for this activity.

Peptide methionine sulfoxide reductase contributes to the maintenance of adhesins in three major pathogens

Pathogenic bacteria rely on adhesins to bind to host tissues. Therefore, the maintenance of the functional properties of these extracellular macromolecules is essential for the pathogenicity of these microorganisms. We report that peptide methionine sulfoxide reductase (MsrA), a repair enzyme, contributes to the maintenance of adhesins in Streptococcus pneumoniae, Neisseria gonorrhoeae, and Escherichia coli. A screen of a library of pneumococcal mutants for loss of adherence uncovered a MsrA mutant with 75% reduced binding to GalNAcbeta1-4Gal containing eukaryotic cell receptors that are present on type II lung cells and vascular endothelial cells. Subsequently, it was shown that an E. coli msrA mutant displayed decreased type I fimbriae-mediated, mannose-dependent, agglutination of erythrocytes. Previous work [Taha, M. K., So, M., Seifert, H. S., Billyard, E. & Marchal, C. (1988) EMBO J. 7, 4367-4378] has shown that mutants with defects in the pilA-pilB locus from N. gonorrhoeae were altered in their production of type IV pili. We show that pneumococcal MsrA and gonococcal PilB expressed in E. coli have MsrA activity. Together these data suggest that MsrA is required for the proper expression or maintenance of functional adhesins on the surfaces of these three major pathogenic bacteria.

The Neisseria transcriptional regulator PilA has a GTPase activity

The pilA gene of Neisseria gonorrhoeae encodes the response regulator of a two-component regulatory system that controls pilin gene expression. Examination of the primary sequence of PilA indicates that the protein contains at least two functional domains. The N-terminal region has a proposed helix-turn-helix motif thought to be involved in DNA binding. This region also contains the residues that are presumed to form the acidic pocket involved in phosphorylation by PilB, the sensor kinase of the system. The C terminus of the protein has extensive homology to the G (GTP-binding) domains of the eukaryotic signal recognition particle (SRP) 54-kDa protein and the alpha subunit of the SRP receptor, or docking protein. This homology also extends to similar regions of the bacterial SRP homologs Ffh and FtsY. Here, we demonstrate that purified PilA has significant GTPase activity, and that this activity has an absolute requirement for MgCl2 and is sensitive to KCl and low pH. We also show that PilA has a strict specificity for GTP, and that GTP hydrolysis follows first order kinetics, with a maximum velocity (Vmax) of 1900 pmol of Pi produced per min per mg of protein and a Km for GTP of 9.6 microM at 37 degrees C.