The Bacillus subtilis gene for the developmental transcription factor σK is generated by excision of a dispensable DNA element containing a sporulation recombinase gene

Abstract

The structural gene (sigK) for the mother-cell RNA polymerase σ-factor σK in Bacillus subtilis is a composite of two truncated genes, named spoIVCB ana spoIIIC, which are brought together by site-specific recombination during sporulation. We now show that the recombination event is compartmentalized in that the mother cell, but not the forespore chromosome, undergoes rearrangement. We also show that spoIIIC (encoding the carboxy-terminal portion of σK) lies ∼42 kb downstream of spoIVCB (encoding the amino-terminal portion) and that the joining of the truncated coding sequences is a reciprocal recombination event in which intervening DNA is deleted from the chromosome as a circle. The rearrangement is governed by the product of a gene named spoIVCA located in the excised DNA, as demonstrated by the observations (1) that the product of spoIVCA, but not the product of any other stage-IV sporulation gene tested, is required for the rearrangement, and (2) that the presence of a cloned copy of the rearranged sigK gene in the chromosome bypasses the requirement for the spoIVCA gene product in sporulation. Because cells engineered to contain an intact copy of sigK sporulate normally, we conclude that the sigK rearrangement is not essential for the control of gene expression during sporulation, and we infer the existence of an additional mechanism for restricting σK-directed transcription to the mother-cell chamber of the sporangium. Finally, the construction of a strain deleted for the entire sigK intervening sequence shows that the 42-kb element contains no genes essential for viability.