Abstract
The plastid genome contains more than one hundred genes and the expression is regulated depending on plastid types and environmental conditions. Sigma factors determine the specificity of the eubacteria-type plastid RNA polymerase (PEP), and the roles of sigma factors for chloroplast gene expression remain largely unknown. In this study, we analyzed the expression of the sigma factors (SIG1-6) under various stress conditions, and found that the SIG5 transcript was rapidly and drastically induced under various stresses conditions. Because this induction was well correlated with the activation of the light-responsive promoter (LRP) of psbD, and because the psbD-LRP activation was abolished in the newly identified sig5 mutant (sig5-2), we conclud that SIG5 is a stress-responsive sigma factor in Arabidopsis, and responsible for the psbD-LRP transcription. We also found that the restoration of PSII activity after high light irradiation was delayed in the sig5 mutant, suggesting roles of SIG5 under stress conditions.
