sigmafactor

Sigma factor, or σ factor, is a protein component of bacterial RNA polymerase that directs transcription initiation by providing promoter specificity. In bacteria, the RNA polymerase core enzyme requires a sigma factor to recognize promoter sequences and to begin RNA synthesis.

When bound to a sigma factor, the RNA polymerase forms the holoenzyme, which binds to promoter elements typically located at about -35 and -10 nucleotides upstream of the transcription start site. The holoenzyme melts the DNA to form the open complex, begins RNA synthesis, and then the sigma factor is released after the first few nucleotides have been made, allowing the core enzyme to proceed with elongation.

Diversity and regulation: Most bacteria encode multiple sigma factors. The major, or housekeeping, sigma factor directs transcription of essential genes during normal growth (e.g., sigma-70 in Escherichia coli). Alternative sigma factors reprogram transcription in response to stress, stationary phase, sporulation, or specific metabolic cues (examples include sigma-32, sigma-S, and sigma-54). Some sigma factors are controlled by anti-sigma factors that sequester them until activation signals are received, providing a means to rapidly adjust gene expression.

Significance: Sigma factors enable rapid transcriptional reprogramming and adaptation to environmental changes. They are central to bacterial stress responses and development, and are studied as potential targets for antimicrobial strategies as well as tools in synthetic biology to control gene expression.