1) RR mail cars 
Sigma factors are proteins that regulate transcription in bacteria. Sigma factors can be activated in response to different environmental conditions. The gene rpoS (RNA polymerase, sigma S) encodes sigma-38 (σ38, or RpoS), a 37.8 kD protein in Escherichia coli. rpoS is transcribed in late exponential phase, and RpoS is the primary regulator of st...