Plastid sigma factors: Their individual functions and regulation in transcription☆

article i nfo Sigma factors are the predominant factors involved in transcription regulation in bacteria. These factors can re- cruit the core RNA polymerase to promoters with specific DNA sequences and initiate gene transcription. The plastids of higher plants originating from an ancestral cyanobacterial endosymbiont also contain sigma factors that are encoded by a small family of nuclear genes. Although all plastid sigma factors contain sequences con- served in bacterial sigma factors, a considerable number of distinct traits have been acquired during evolution. The present review summarises recent advances concerning the regulation of the structure, function and activity of plastid sigma factors since their discovery nearly 40 years ago. We highlight the specialised roles and overlap- ping redundant functions of plastid sigma factors according to their promoter selectivity. We also focus on the mechanisms that modulate the activity of sigma factors to optimise plastid function in response to developmen- tal cues and environmental signals. This article is part of a Special Issue entitled: Chloroplast Biogenesis.