Protein level identification of the Listeria monocytogenes Sigma H, Sigma L, and Sigma C regulons

BACKGROUND: Transcriptional regulation by alternative sigma (σ) factors represents an important mechanism that allows bacteria to rapidly regulate transcript and protein levels in response to changing environmental conditions. While the role of the alternative σ factor σ(B) has been comparatively well characterized in L. monocytogenes, our understanding of the roles of the three other L. monocytogenes alternative σ factors is still limited. In this study, we employed a quantitative proteomics approach using Isobaric Tags for Relative and Absolute Quantitation (iTRAQ) to characterize the L. monocytogenes σ(L), σ(H), and σ(C) protein regulons. Proteomic comparisons used a quadruple alternative σ factor mutant strain (ΔBCHL) and strains expressing a single alternative σ factor (i.e., σ(L), σ(H), and σ(C); strains ΔBCH, ΔBCL, and ΔBHL) to eliminate potential redundancies between σ factors. RESULTS: Among the three alternative σ factors studied here, σ(H) provides positive regulation for the largest number of proteins, consistent with previous transcriptomic studies, while σ(L) appears to contribute to negative regulation of a number of proteins. σ(C) was found to regulate a small number of proteins in L. monocytogenes grown to stationary phase at 37°C. Proteins identified as being regulated by multiple alternative σ factors include MptA, which is a component of a PTS system with a potential role in regulation of PrfA activity. CONCLUSIONS: This study provides initial insights into global regulation of protein production by the L. monocytogenes alternative σ factors σ(L), σ(H), and σ(C). While, among these σ factors, σ(H) appears to positively regulate the largest number of proteins, we also identified PTS systems that appear to be co-regulated by multiple alternative σ factors. Future studies should not only explore potential roles of alternative σ factors in activating a “cascade” of PTS systems that potentially regulate PrfA, but also may want to explore the σ(L) and σ(C) regulons under different environmental conditions to identify conditions where these σ factors may regulate larger numbers of proteins or genes.