科研之友
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摘要
Gene expression during spore development in Bacillus subtilis is controlled by cell type-specific RNA polymerase sigma factors. sigma(F) and sigma(E) control early stages of development in the forespore and the mother cell, respectively. When, at an intermediate stage in development, the mother cell engulfs the forespore, sigma(F) is replaced by sigma(G) and sigma(E) is replaced by sigma(K). The anti-sigma factor CsfB is produced under the control of sigma(F) and binds to and inhibits the auto-regulatory sigma(G), but not sigma(F). A position in region 2.1, occupied by an asparagine in sigma(G) and by a glutamate in o(F), is sufficient for CsfB discrimination of the two sigmas, and allows it to delay the early to late switch in forespore gene expression. We now show that following engulfment completion, csfB is switched on in the mother cell under the control of sigma(K) and that CsfB binds to and inhibits sigma(E) but not sigma(K), possibly to facilitate the switch from early to late gene expression. We show that a position in region 2.3 occupied by a conserved asparagine in sigma(E) and by a conserved glutamate in sigma(K) suffices for discrimination by CsfB. We also show that CsfB prevents activation of sigma(G) in the mother cell and the premature sigma(G)-dependent activation of sigma(K). Thus, CsfB establishes negative feedback loops that curtail the activity of sigma(E) and prevent the ectopic activation of sigma(G) in the mother cell. The capacity of CsfB to directly block sigma(E) activity may also explain how CsfB plays a role as one of the several mechanisms that prevent sigma(E) activation in the forespore. Thus the capacity of CsfB to differentiate between the highly similar sigma(F)/sigma(G) and sigma(E)/sigma(K) pairs allows it to rinforce the cell-type specificity of these sigma factors and the transition from early to late development in B. subtilis, and possibly in all sporeformers that encode a CsfB orthologue.
- 出版日期2015-3
