Involvement of Stringent Factor RelA in Expression of the Alkaline Protease Gene aprE in Bacillus subtilis

Expression of Bacillus subtilis aprE, encoding an extracellular alkaline protease, is positively regulated by phosphorylated DegU, the regulator of a two-component regulatory system, DegS-DegU. We found that the expression of an aprE'-'lacZ fusion was greatly reduced in a disruption mutant with a mutation of relA, which encodes the stringent factor RelA. The level of DegU in the relA mutant was similar to that in the wild-type cell. A relA degU double mutation did not result in a further decrease of the aprE'-'lacZ level found in a degU single mutant. The expression of the aprE'-'lacZ fusion in the relA mutant was stimulated by multicopy degR or the degU32(Hy) and degS200(Hy) mutations that cause the stabilization of phosphorylated DegU. Furthermore, the expression of sacB'-'lacZ, which is also dependent on phosphorylated DegU, was stimulated by the relA mutation, and this stimulation was not seen in the relA degU double mutant. These results show that RelA (or its product guanosine-3',5'-bisdiphosphate [pp Gpp]) does not affect the phosphorylation of DegU and suggest that it participates in the expression of aprE and sacB through the regulation of DegU-dependent transcription.