In bacteria, what is the role of sigma factors?

• A. Sigma factors direct RNA polymerase to promoters enabling transcription initiation
• B. Sigma factors catalyze RNA synthesis
• C. Sigma factors unwind DNA
• D. Sigma factors ligate nucleotides

Answer: (A)

Sigma factors in bacteria guide RNA polymerase to promoter regions to initiate transcription. They are part of the RNA polymerase holoenzyme and play the crucial role of recognizing promoter sequences, helping the core enzyme bind specifically to the start sites of genes. By interacting with the -35 and -10 promoter elements, sigma factors position the polymerase correctly and help melt the DNA to form the open complex, enabling transcription to begin. After initiation starts, the sigma factor typically dissociates, allowing the core RNA polymerase to proceed with elongation.

This is distinct from other roles: sigma factors do not synthesize RNA themselves—that’s the job of the RNA polymerase core enzyme. They do not function as helicases to unwind DNA directly, and they do not ligate nucleotides; those activities belong to other enzymes involved in replication or repair. Different sigma factors can be used by bacteria to respond to environmental changes, directing the expression of specific sets of genes as needed.