Identification of a second Listeria secA gene associated with protein secretion and the rough phenotype.

We describe the identification and characterization of a second secA gene in Listeria monocytogenes. This gene, termed secA2, is involved in smooth-rough phenotypic variation and secA2 expression contributes to bacterial virulence. Spontaneous rough (R-) variants of L. monocytogenes grow in chains and form rough colonies on solid media. A subset of R-variants, classified here as type I, also shows reduced secretion of an autolysin, p60. We find that disruptions and in frame deletions in secA2 confer phenotypes identical to those of spontaneous type I R-variants. Additionally, the secA2 genes from two spontaneous type I R-variants encoded truncated SecA2 proteins. Mutations were not found in the secA2 genes from the remaining five independent R-variants, four of which showed a distinct (type II) rough morphology and secreted wild-type levels of p60. Expression of an epitope-tagged SecA2 in the DeltasecA2 strain and a spontaneous R-variant restored normal cell septation and smooth colony morphology. These data suggest that mutations in both secA2 and other genes contribute to smooth-rough phase variation in L. monocytogenes. Expression of the full-length SecA2 also promotes secretion of p60 and a set of additional L. monocytogenes proteins. We hypothesize that SecA2-dependent protein secretion plays a role in the colonization of environmental and host surfaces.