Sigma Holds On

Abstract

Many decades of work have provided details of the molecular mechanism and machinery needed for prokaryotic gene expression. Bacterial genes are transcribed by the RNA polymerase enzyme, which associates with a sigma subunit during transcription initiation. Afterwards, the sigma subunit is released from the polymerase and an elongation factor, NusA, binds. NusA remains bound to the polymerase throughout transcription elongation and termination and is then removed to allow sigma to bind once again for transcription re-initiation. This is termed the “sigma cycle.”