Plastid transcription occurs via either a nucleus-encoded (NEP) or plastid-encoded (PEP) polymerase. PEP requires a nucleus-encoded sigma factor for promoter recognition, represented by a single member in bacteria, but by a small gene family in plants. This study investigated the diversification of sigma factors, and the resultant implications on family member function. Investigation of sigma factor maintenance across 31 plant species showed the consistent presence of multiple (generally 6) sigma factors in angiosperms and supported a mechanism for sigma factor evolution involving late appearance of SIG4-like proteins. Interrogation of arabidopsis sigma factor transcript expression across multiple experimental conditions revealed factor-specific profiles, yet indicated general correlation of transcript accumulation with requirements for chloroplast gene expression. Plastid mRNA profiling was undertaken for arabidopsis mutants with defects in individual sigma factor genes, and revealed previously undefined putative sigma factor activity on multiple gene promoters. Comparative analysis of profiles suggested a level of sigma factor functional redundancy, which emphasised the importance of interpreting sigma factor activity in the context of all family members, and the need to extend future investigations beyond analysis of single mutants. Overall, this study suggested a dominant, but somewhat independent, role for SIG2 and SIG6 in transcription of multiple transcripts, and a more redundant role for SIG3. SIG5 likely acts as a 'responsive' sigma factor and SIG1 seems predominantly involved in responsive regulation of photosystem genes. Finally, a case study suggested the involvement of SIG4 in regulation of the plastid-encoded NAD(P)H dehydrogenase-like complex (NDH) genes, an a role for this factor in coordinated accumulation of the NDH complex.
|Publication status||Unpublished - 2012|