Temporal and selective association of multiple sigma factors with RNA polymerase during sporulation in Bacillus subtilis.
BACKGROUND
During sporulation in Bacillus subtilis, an asymmetric division produces two cells, a forespore and mother cell, with which follow different developmental paths. The highly ordered programme of temporal and spatial gene activation during sporulation is governed by the principal RNA polymerase holoenzyme (EsigmaA) and alternative holoenzyme forms containing the developmental sigma factors sigmaH, sigmaF, sigmaE, sigmaG and sigmaK, which appear successively during development. The control mechanism(s) of temporal and selective association of multiple sigma factors with core RNA polymerase is unclear. As a first step to addressing these issues, this report quantifies the amount of each subunit of RNA polymerase that is present in the sporangium during sporulation, and analyses in vitro the relative affinities of each sigma subunit for core RNA polymerase.


RESULTS
Using quantitative immunoblot analysis, the amounts of EsigmaA, EsigmaH, EsigmaE and EsigmaK in relation to the total amount of RNA polymerase at appropriate time-points were found to be 15%, 1%, 6% and 2%, respectively. Therefore, the core RNA polymerase is predicted to be in excess. The level of core RNA polymerase and sigmaA remained constant during the transition from vegetative growth to sporulation, whereas the sporulation-specific sigma factors appeared successively, in the order sigmaH, sigmaE and sigmaK. Competition experiments between sigma factors in an in vitro transcription system revealed the dominance of sigmaA over sigmaH and sigmaE for open promoter complex formation. These results are inconsistent with the idea that late appearing sigma factors can displace earlier appearing sigmas from the core enzyme.


CONCLUSIONS
As the core RNA polymerase is in excess, the results suggest that successive sigma factors can bind to core RNA polymerase without having to displace earlier appearing sigma factors. Thus, the programme of gene expression during sporulation might not require mechanisms for the substitution of one sigma factor by another on the core RNA polymerase.
Fujita
M
M
Radioisotope Center, National Institute of Genetics, Mishima, Shizuoka 411-8540, Japan. mafujita@lab.nig.ac.jp
D001412:Bacillus subtilis / Q000502:physiology*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D001667:Binding, Competitive; D012321:DNA-Directed RNA Polymerases / Q000235:genetics / Q000378:metabolism*; D015151:Immunoblotting; D009154:Mutation; D012808:Sigma Factor / Q000235:genetics / Q000378:metabolism*; D013171:Spores, Bacterial / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic
D001426:Bacterial Proteins; D012808:Sigma Factor; C087737:Spo0H protein, Bacillus subtilis; D014157:Transcription Factors; C063202:sporulation-specific sigma factors; D012321:DNA-Directed RNA Polymerases