Abstract
Chloroplast contains a eubacteria-like multi subunit RNA polymerase (PEP) that is responsible for transcription of plastid-encoded photosynthesis-related genes. Exchangeable nuclear-encoded sigma factors confer promoter specificity of PEP. In bacteria, most of the housekeeping genes are transcribed by the RNA polymerase containing primary sigma factors, while alternative sigma factors are involved in transcription of stress responsive genes. Arabidopsis encodes six chloroplast sigma factors. However, general sigma factors important for transcription of standard photosynthesis-related genes have not been identified in chloroplasts. The AtSIG6 null mutant shows pale green phenotypes in early cotyledons. Accumulation of most PEP-dependent transcripts was diminished in young seedlings of the sig6 mutant. All of the AtSig6-dependent genes are preceded by the σ70-type promoters. We concluded that AtSig6 is one of the general sigma factors in chloroplasts that recognize σ70-type promoters and it plays an important role in early chloroplast development in cotyledons.
