Single-Stranded DNA Binding Protein

Abstract

Bacteriophage P1 encodes a single-stranded DNA-binding protein (SSB-P1), which shows 66% amino acid sequence identity to the SSB protein of the host bacterium Escherichia coli. A phylogenetic analysis indicated that the P1 ssb gene coexists with its E. coli counterpart as an independent unit and does not represent a recent acquirement of the phage. The P1 and E. coli SSB proteins are fully functionally interchangeable. SSB-P1 is nonessential for phage growth in an exponentially growing E. coli host, and it is sufficient to promote bacterial growth in the absence of the E. coli SSB protein. Expression studies showed that the P1 ssb gene is transcribed only, in an rpoS-independent fashion, during stationary-phase growth in E. coli. Mixed infection experiments demonstrated that a wild-type phage has a selective advantage over an ssb-null mutant when exposed to a bacterial host in the stationary phase. These results reconciled the observed evolutionary conservation with the seemingly redundant presence of ssb genes in many bacteriophages and conjugative plasmids.Single-stranded DNA-binding proteins (SSBs) have been found in all investigated organisms from bacteria to humans (7,36). These proteins have no enzymatic activity, but as their name indicates, they bind nonspecifically and stoichiometrically to single-stranded DNA, hold the single strands in an open conformation, and protect them from degradation by nucleases. In Escherichia coli, the assembly and propagation of the DNA replication fork, RecA-mediated homologous recombination, and the DNA base excision repair mechanism depend on the presence of SSB (see Chase [6], Meyer and Laine [29], and Lohmann and Ferrari [26] for reviews on E. coli SSB). SSB is an essential protein, and a functional copy of an ssb gene is present in every viable bacterial cell. Despite this fact, many E. coli bacteriophages and conjugative plasmids encode their own SSBs (9). Most of these episomal SSBs show strong conservation of key residues important for SSB function (24), and among them, several were shown to be able to reverse the temperature-sensitive growth defect of an ssb-1 mutant E. coli strain (9,10,12,24,29,34). The exact function of the episomal SSBs, however, was unclear, and the fact that most of them appeared to be dispensable remained puzzling (7,10,12).Under harsh natural conditions, bacteria more often than not encounter limited resources allowing only minimal or no growth (33). E. coli drastically adapts the expression of its genetic information in order to endure and survive such periods of stationary-phase growth (17, 48). The response of bacteriophages to such growth conditions varies (1). Some phages are unable to grow or acquire a stationary-phase dormant state, called pseudolysogeny (37), and resume growth once the host cells obtain fresh nutrients (21, 37). Others seem to be able to grow, albeit with reduced efficiency compared to exponential growth conditions (19,42). Large bacteriophages like T4 (20) or P1 (47) contain a plethora of genes which are not absol...