Cross Talk between SigB and PrfA in Listeria monocytogenes Facilitates Transitions between Extra- and Intracellular Environments

Abstract

The foodborne pathogen Listeria monocytogenes can modulate its transcriptome and proteome to ensure its survival during transmission through vastly differing environmental conditions. While L. monocytogenes utilizes a large array of regulators to achieve survival and growth in different intra- and extrahost environments, the alternative sigma factor σ^B and the transcriptional activator of virulence genes protein PrfA are two key transcriptional regulators essential for responding to environmental stress conditions and for host infection. Importantly, emerging evidence suggests that the shift from extrahost environments to the host gastrointestinal tract and, subsequently, to intracellular environments requires regulatory interplay between σ^B and PrfA at transcriptional, posttranscriptional, and protein activity levels. Here, we review the current evidence for cross talk and interplay between σ^B and PrfA and their respective regulons and highlight the plasticity of σ^B and PrfA cross talk and the role of this cross talk in facilitating successful transition of L. monocytogenes from diverse extrahost to diverse extra- and intracellular host environments.

Keywords: Listeria monocytogenes; PrfA; gene regulation; general stress response; sigma B; virulence.

FIG 1

Cross talk between the regulatory circuits controlled by the σ^B and PrfA regulons. σ^B regulates genes involved in stress response, and PrfA activates virulence-related genes. Although σ^B and PrfA coregulate some genes involved in bile resistance and internalization, σ^B modulates PrfA activity through direct transcriptional activation from the prfAP2 promoter and through indirect posttranscriptional repression under some environmental conditions, e.g., when glucose is transferred into the cell by the PTS^Man system. prfA expression is activated through an autoregulatory mechanism and by the branched-chain amino acid sensor CodY; prfA translation is repressed by the noncoding RNA SreA.

FIG 2

Organization of the plcA-prfA locus. (A) Operon structure and promoter arrangement of the plcA-prfA locus. PrfA is expressed as monocistronic RNA from the prfAP1 and prfAP2 promoters and as bicistronic RNA from the prfAP3 promoter. (B) DNA sequence of the prfAP1 and prfAP2 promoters shows an extensive overlap between σ^B (blue), σ^A (red), and PrfA recognition motifs. Double-headed arrows denote regions in prfAP2 that were deleted in several studies (51, 121).

FIG 3

(A, B) PrfA structure in the absence (A) and presence (B) of GSH (136). (C) DNA-induced bending by Crp-cAMP complex (202).

FIG 4

Model for possible interactions among the Clp system, PrfA, and σ^B. PrfA regulon expression is reduced in the absence of several members of the Clp system, including ClpP, ClpX, ClpE, and YjbH (LMO0964). The programmed-protein-degradation Clp system is positively regulated by σ^B and repressed by CtsR; σ^B downregulates PrfA activity through posttranscriptional regulation. The Clp system may exert its effect on PrfA regulon expression through degradation of an as-yet-unidentified factor that inhibits PrfA expression or activity.

FIG 5

Model for carbon source-induced repression of PrfA activity. In the presence of a carbon source transported by PTS systems, e.g., glucose, σ^B activates expression of the PTS^Mpo uptake system. Transport of glucose across the membrane results in dephosphorylation of EIIB^Mpo, relieving repression of ManR, which in turn, activates expression of the PTS^Man uptake system. Uptake of glucose using the PTS^Man system results in accumulation of nonphosphorylated EIIAB^Man, which inhibits PrfA activity, perhaps by direct protein sequestration (164).

FIG 6

L. monocytogenes modulates relative expression levels of the PrfA and σ^B regulons in saprophytic, digestive tract, and intracellular environments to enhance fitness and ensure infection. Outside the host, and in the absence of stress, L. monocytogenes σ^B is inactive due to its sequestration by the antisigma RsbW (for a recent review, see reference 167); exposure to environmental stresses results in release of σ^B by RsbW, which enables it to bind to core RNA polymerase and to become active. In the saprophytic environment, the PrfA regulon is repressed through multiple transcriptional and posttranscriptional mechanisms that control prfA expression and PrfA levels and activity. After ingestion of L. monocytogenes by the host, σ^B is a major contributor to bacterial survival under the harsh conditions encountered in the GI tract; prfA mRNA is reported to be present at less than one copy per cell (37, 54). Graded regulation of the PrfA regulon is predominantly responsible for appropriate temporal and spatial expression of different virulence-related genes in the host. Inside the host cell, PrfA is highly induced; it shifts to its active form by binding to its cofactor. PrfA-GSH activates the expression of genes encoding different virulence-related functions. Although σ^B’s contributions to L. monocytogenes survival within the host cell are not well defined, it is plausible that σ^B may be involved in responses to host-induced stresses.