Three sporulation-specific genes (orfA, sigE, sigG) from Clostridium acetobutylicum ATCC 824 are arranged in a cluster, encoding the putative sigma(E)-processing enzyme, sigma(E), and sigma(G) respectively. When they were transformed into Clostridium acetobutylicum while on a plasmid functional in this organism, transformants did not survive. Three kinds of recombinations were then attempted with nonreplicative plasmids: duplication of orfA and sigE, replacement of all of the three genes, and inactivation of orfA. While the wild-type strain ceased to grow and produce solvents in batch cultures after approximately 24 h, mutant strains were isolated that showed sustained growth for a much longer time and produced a threefold increase in acetone and butanol in test tube cultures. In addition, one of the derived strains showed a significantly higher growth rate, Features of the restriction maps of the recombinants did not correlate with expected maps, indicating possible complications occurring during the recombination events.