The nucleotide sequence of the sporulation gene spolVC cisA in BacUlus subtilis was determined and found to encode a protein of 500 amino acid residues with a calculated molecular weight of 57,481, which It is well known that DNA rearrangement occurs in some cases during cellular differentiation. The best known developmentally regulated rearrangements are those of immunoglobulin genes in vertebrates (24). Since endospore formation of Bacillus subtilis is a simple form of cellular differentiation, it has been speculated for a long time that DNA rearrangement occurs during sporulation (14). Sporulation of B. subtilis initiates in response to nutrient deprivation. In the late stage of sporulation (stages 3 to 7), the cells are divided into two compartments, mother cell and forespore. Each compartment contains a single intact chromosome. The forespore becomes the mature spore, but the mother cell lyses after maturation.The sporulation gene spoIVC of B. subtilis is located in a 3.6-kilobase (kb) EcoRI fragment cloned in temperate bacteriophage 4105 (6). Farquhar and Yudkin (5) described a series of new spoIVC sporulation mutations and suggested that all the mutations fall into two cistrons, cisA and cisB. We found that spoIVC133 is a mutation in the PvuII B fragment using a transformation experiment (6) and that this mutation is located in the cisA gene (see Fig. 1). Kunkel et al. (16) suggested that spoIVC23 is located in cisB and that cisB is expressed from 3.5 h after the initiation of sporulation (T3.5).Recently, Stragier et al. (23) showed that spoIVC cisB and spoIIIC (4) are fused at T3 as a result of DNA rearrangement and that the fused gene encodes the late-sporulation-specific sigma factor K (CrK) (15).We show here that spoIVC cisA encodes a protein whose N-terminal region is highly homologous to DNA recombinases and that the DNA rearrangement does not occur in spoIVC cisA mutants. These results suggest that the spoIVC cisA protein is a site-specific DNA recombinase which rearranges mother cell DNA during sporulation.