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σB, a secondary sigma factor of Bacillus subtilis, was found to increase 5- to 10-fold when cultures were shifted from 37 to 48°C. Western blot (immunoblot) analyses, in which monoclonal antibodies specific for the sigB operon products RsbV, RsbW, and σB were used to probe extracts from wild-type and mutant B. subtilis strains, revealed that all three proteins increased coordinately after heat shock and that this increase was dependent on σB but not RsbV, a positive regulator normally essential for σB-dependent sigB expression. Nuclease protection experiments of RNA synthesized after heat shock supported the notion that the shift to 48°C enhanced transcription from the sigB operon's σB-dependent promoter. The level of mRNA initiating at the σB-dependent ctc promoter was also seen to increase approximately 5- to 10-fold after heat shock. Pulse-labeling of the proteins synthesized after a shift to 48°C demonstrated that sigB wild-type and mutant strains produced the maijor heat-inducible proteins in similar amounts; however, at least seven additional proteins were present after the temperature shift in the wild-type strain but absent in the sigB null mutant. Thus, although σB is not required for the expression of essential heat shock genes, it is activated by heat shock to elevate its own synthesis and possibly the synthesis of several other heat-inducible proteins.