Listeria monocytogenes σH contributes to expression of competence genes and intracellular growth

Abstract

The alternative sigma factor σH has two functions in Gram-positive bacteria: It regulates sporulation and the development of genetic competence. Listeria monocytogenes is a nonsporulating species in which competence has not yet been detected. Nevertheless, the main competence regulators and a series of orthologous genes that form the competence machinery are present in its genome; some of the competence genes play a role in optimal phagosomal escape. In this study, strains overexpressing σH and strains with a σH deletion were used to elucidate the contribution of σH to the expression of the competence machinery genes in L. monocytogenes. Gene expression analysis showed that σH is, indeed, involved in comG and comE regulation. Unexpectedly, we observed a unique regulation scheme in which σH and the transcription factor ComK were involved. Population-level analysis showed that even with the overexpression of both factors, only a fraction of the cells expressed the competence machinery genes. Although we could not detect competence, σH was crucial for phagosomal escape, which implies that this alternative sigma factor has specifically evolved to regulate the L. monocytogenes intracellular life cycle.