σ B - and PrfA-Dependent Transcription of Genes Previously Classified as Putative Constituents of the Listeria monocytogenes PrfA Regulon

Mounting evidence suggests that σ^B and PrfA coregulate transcription of multiple genes in Listeria monocytogenes, therefore, the relative contributions of σ^B and PrfA to transcript levels of genes identified previously as differentially regulated by PrfA were measured. Group I genes are recognized virulence genes that are positively regulated by PrfA; group II genes were reported previously as negatively regulated by PrfA; and multiple group III genes were proposed to be coregulated by σ^B and PrfA. Transcript levels for selected genes were measured by quantitative reverse transcriptase polymerase chain reaction (RT-PCR) in L. monocytogenes 10403S as well as in otherwise isogenic ΔsigB, ΔprfA, and ΔsigBΔprfA strains grown under conditions demonstrated to induce either PrfA activity (0.2% activated charcoal) or both PrfA and σ^B activity (stationary phase). Although the Group I gene plcA was positively regulated by PrfA, transcript levels for the group II genes lmo0278 and lmo0178 were not affected by the prfA deletion. While the sigB deletion significantly affected transcript levels for the selected group III genes (i.e., lmo0596, lmo0654, bsh, and opuCA), with lower transcript levels in the ΔsigB strains under all conditions tested, transcript levels for these genes were not significantly affected by the prfA deletion. Our results suggest that the regulatory interactions between PrfA and σ^B contribute to PrfA's predominant role as a direct regulator of virulence genes critical for invasion and intracellular survival in L. monocytogenes 10403S, while σ^B regulates a wider range of virulence and stress response genes.