Practice question

Alternative sigma factors redirect RNA polymerase holoenzyme to distinct promoter classes under stress conditions. Sigma32 encoded by rpoH in Escherichia coli accumulates during temperature upshift because heat-induced misfolded proteins titrate chaperones DnaK, DnaJ, GroEL, releasing sigma32 from sequestration and from FtsH-mediated proteolysis. Free sigma32 binds core polymerase, activating heat-shock promoters defined by -35 CTTGAA and -10 CCCCATNT consensus driving transcription of chaperones GroEL, DnaK, proteases ClpB, Lon, and repair proteins. This rapid reprogramming restores proteostasis, increases refolding capacity, and provides transient thermotolerance until homeostasis returns and sigma32 degraded via feedback regulation.