Abstract
Alternative sigma factors have led the core RNA polymerase (RNAP) to recognize different sets of promoters to those recognized by the housekeeping sigma A-directed RNAP. This change in RNAP promoter selectivity allows a rapid and flexible reformulation of the genetic program to face environmental and metabolic stimuli that could compromise bacterial fitness. The model bacterium Bacillus subtilis constitutes a matchless living system in the study of the role of alternative sigma factors in gene regulation and physiology. SigB from B. subtilis was the first alternative sigma factor described in bacteria. Studies of SigB during the last 40 years have shown that it controls a genetic universe of more than 150 genes playing crucial roles in stress response, adaption, and survival. Activation of SigB relies on three separate pathways that specifically respond to energy, environmental, and low temperature stresses. SigB homologs, present in other Gram-positive bacteria, also play important roles in virulence against mammals. Interestingly, during recent years, other unexpected B. subtilis responses were found to be controlled by SigB. In particular, SigB controls the efficiencies of spore and biofilm formation, two important features that play critical roles in adaptation and survival in planktonic and sessile B. subtilis communities. In B. subtilis, SigB induces the expression of the Spo0E aspartyl-phosphatase, which is responsible for the blockage of sporulation initiation. The upregulated activity of Spo0E connects the two predominant adaptive pathways (i.e., sporulation and stress response) present in B. subtilis. In addition, the RsbP serine-phosphatase, belonging to the energy stress arm of the SigB regulatory cascade, controls the expression of the key transcription factor SinR to decide whether cells residing in the biofilm remain in and maintain biofilm growth or scape to colonize new niches through biofilm dispersal. SigB also intervenes in the recognition of and response to surrounding microorganisms, a new SigB role that could have an agronomic impact. SigB is induced when B. subtilis is confronted with phytopathogenic fungi (e.g., Fusarium verticillioides) and halts fungal growth to the benefit of plant growth. In this article, we update and review literature on the different regulatory networks that control the activation of SigB and the new roles that have been described the recent years.
Highlights
SigB AS A MODEL OF ALTERNATIVE SIGMA FACTOR PRESENT IN BACTERIAGene expression is a fundamental process that is conserved from bacteria to humans. The first step in gene expression is transcription, which is performed by structurally conserved DNA-dependent RNA polymerases (RNAPs), resulting in the synthesis of an RNA molecule from a DNA template
Specialty section: This article was submitted to Microbial Physiology and Metabolism, a section of the journal Frontiers in Microbiology
Despite the high SigB activity of B. subtilis growing at low temperature, the sporulation proficiency is extremely low, less than 1% for cultures grown at 17–20◦C, compared to a nearly 100% of sporulation efficiency for cultures grown at 37◦C (Brigulla et al, 2003; Méndez et al, 2004)
Summary
Gene expression is a fundamental process that is conserved from bacteria to humans. The first step in gene expression is transcription, which is performed by structurally conserved DNA-dependent RNA polymerases (RNAPs), resulting in the synthesis of an RNA molecule from a DNA template. The exact role of the ω subunit is unclear, but it is related in both structure and sequence to the eukaryotic polymerase subunit Rpb (Minakhin et al, 2001) It appears to be responsible for controlling transcription in response to nutrient changes, correct folding of the β’ subunit, and its assembly into the core multi-subunit enzyme (Mukherjee et al, 1999; Vrentas et al, 2005; Chatterji et al, 2007). In the model bacteria Escherichia coli and Bacillus subtilis, the most important housekeeping sigma factors are Sig and SigA, respectively They are present during planktonic and sessile growth (Losick and Pero, 1981; Gitt et al, 1985) and are responsible for initiating transcription from most promoters under optimal physiological conditions.
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