Sigma factors also contribute to DNA strand separation, which is a critical step in transcription initiation. The sigma subunit dissociates from the RNA polymerase core enzyme shortly after transcription begins, thus becoming available for reassociation. Associations between different alternative sigma factors and core RNA polymerase essentially reprogram the ability of the RNA polymerase holoenzyme to recognize different promoter sequences and express entirely new sets of target genes. As the set of genes controlled by a single sigma factor (also known as the regulon) can number in the hundreds, sigma factors provide effective mechanisms for simultaneously regulating large numbers of prokaryotic genes.