Two separable functional domains in the sigma-subunit of RNA polymerase in Bacillus subtilis?

The sigma-subunit of RNA polymerase is responsible for promoter recognition in prokaryotes [(1969) Nature 221, 43-46]. Alterations in the sigma-subunit are thought to be involved in controlling 'global' changes in gene expression, such as those involved in differentiation in the spore-forming bacterium Bacillus subtilis [(1981) Cell 25, 582-584]. Stragier et al. [(1985) FEBS Lett. 195, 3-11] have proposed that sigma-factors are composed of two domains: a C-terminal domain involved in promoter recognition and an N-terminal domain involved in interactions with RNA polymerase. We have sequenced another developmental gene from B. subtilis, spoIIIC, and the strong homology of its predicted product suggests that it too may be a sigma-factor. However, the spoIIIC product is small and lacks completely the conserved N-terminal domain of the sigma-subunits. I propose that the product of the spoIIIC gene may carry out the DNA-recognition functions of a sigma-factor but that it probably requires an auxiliary factor to interact with core RNA polymerase.