Evolution of a novel gene pair from a canonical toxin-antitoxin module in Escherichia coli

Free-living bacteria are continuously subjected to environmental stress. This stress can be in the form of a change in temperature, pH, osmolarity or nutritional starvation. Most bacterial species contain gene modules known as Toxin-Antitoxin (TA) systems that reversibly inhibit cellular growth in response to stress; thereby helping the cells cope with a changing environment. One mechanism that bacteria have developed to combat fluctuations in environmental temperature is the cold-shock response. This response helps exponentially growing cells buffer themselves against a downshift in temperature from their optimal growing temperature; typically a shift from 37??C to 15??C for Escherichia coli (E. coli). Cold-shock proteins (Csp) are synthesized at this time. Protein Y (PY), the protein product of gene yfiA in E. coli is suggested to be a cold-shock related protein. It prevents ribosomes from dissociation during cold-shock, and in stationary phase, thereby blocking translational elongation and inhibiting cell growth. This mechanism resembled that of a typical TA system toxin. We identified a small gene, b2596, upstream of yfiA and propose that the b2596-yfiA module evolved from a true proteic TA system that functioned in cold-shock conditions; Protein X (PX), product of b2596, being the antitoxin and PY the toxin. The module still retains some of its TA system characteristics: both genes encode small proteins, have opposing charges and show sequence similarity to known TA genes. Also, like a true TA system b2596, the proposed antitoxin gene, precedes yfiA, the proposed toxin gene. However, we found that the two genes have independent transcriptional start sites. Also b2596 encodes a leaderless mRNA with UUG start and thus we predict that it cannot be translated well in vivo. PY inhibits growth of E. coli cells and functions in helping the bacterial population to survive cold-shock. Our data suggest that b2596 and yfiA have evolved from a canonical proteic TA module that was functional in cold shock. The two genes are now independent and responsive to cold shock.M.S.Includes bibliographical references (p. 56-60)