Reduced capacity of alternative sigmas to melt promoters ensures stringent promoter recognition.
In bacteria, multiple sigmas direct RNA polymerase to distinct sets of promoters. Housekeeping sigmas direct transcription from thousands of promoters, whereas most alternative sigmas are more selective, recognizing more highly conserved promoter motifs. For sigma(32) and sigma(28), two Escherichia coli Group 3 sigmas, altering a few residues in Region 2.3, the portion of sigma implicated in promoter melting, to those universally conserved in housekeeping sigmas relaxed their stringent promoter requirements and significantly enhanced melting of suboptimal promoters. All Group 3 sigmas and the more divergent Group 4 sigmas have nonconserved amino acids at these positions and rarely transcribe >100 promoters. We suggest that the balance of "melting" and "recognition" functions of sigmas is critical to setting the stringency of promoter recognition. Divergent sigmas may generally use a nonoptimal Region 2.3 to increase promoter stringency, enabling them to mount a focused response to altered conditions.
Koo
Byoung-Mo
BM
Department of Microbiology and Immunology, University of California at San Francisco, San Francisco, California 94158, USA.
Rhodius
Virgil A
VA
Nonaka
Gen
G
deHaseth
Pieter L
PL
Gross
Carol A
CA
D000595:Amino Acid Sequence; D001419:Bacteria / Q000235:genetics / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D009691:Nucleic Acid Denaturation; D010802:Phylogeny; D011401:Promoter Regions, Genetic / Q000235:genetics*; D016415:Sequence Alignment; D012808:Sigma Factor / Q000378:metabolism*
D001426:Bacterial Proteins; D012808:Sigma Factor