Functional Consequences of Genome Evolution inListeria monocytogenes: the lmo0423 and lmo0422 Genes Encode σCand LstR, a Lineage II-Specific Heat Shock System

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Abstract
Listeria monocytogenesstrains belonging to phylogenetic lineage II (serotypes 1/2a, 1/2c, and 3a) carry a lineage-specific genome segment encoding a putative sigma subunit of RNA polymerase (lmo0423, herein referred to assigC), a gene of unknown function (lmo0422) similar to thepadRfamily of regulators, and a gene that is similar to therodA-ftsWfamily of cell wall morphology genes (lmo0421). To understand the function of this set of genes, their expression patterns and the effects of null mutations in the lineage IIL. monocytogenesstrain 10403S were examined. The data are consistent with the three genes comprising an operon (thesigCoperon) that is highly induced by temperature upshift. The operon is transcribed from three different promoters, the proximal of which (P1) depends uponsigCitself. Null mutations insigCor lmo0422 increase the death rate at lethal temperatures and cause loss of thermal adaptive response, whereas the lmo0421 mutation causes only a loss of the adaptive response component. Only thesigCmutation affects transcription from the P1 promoter, whereas ectopic expression of lmo0422 from the PSPACpromoter complements the individual lmo0422 andsigCnull mutations, showing that lmo0422 is the actual thermal resistance regulator or effector whilesigCprovides a mechanism for temperature-dependent transcription of lmo0422 from P1. Our genetic and phylogenetic analyses are consistent with lmo0422—renamedlstR(for lineage-specific thermal regulator)—andsigCcomprising a system of thermal resistance that was ancestral to the genusListeriaand was subsequently lost during divergence of the lineage IL. monocytogenespopulation.

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