Nuclear encoding of a plastid sigma factor in rice and its tissue- and light-dependent expression

A full-length cDNA encoding a putative sigma factor for a plastid RNA polymerase was isolated from the higher plant Oryza sativa . The nucleotide sequence of the corresponding nuclear gene, named Os-sigA ( O.sativa sigma A), predicts a polypeptide of 519 amino acids that contains a putative plastid-targeting sequence in its N-terminal region. The predicted mature protein shows extensive sequence homology to bacterial sigma factors, encompassing the conserved regions 1.2, 2.1, 2.2, 2.3, 2.4, 3, 4.1 and 4.2 implicated in binding to -10 promoter elements, promoter melting and interaction with the core RNA polymerase enzyme. RNA blot analysis revealed that the abundance of Os-sigA transcripts was markedly greater in green shoots than in roots or in dark-grown etiolated shoots of rice seedlings. Furthermore, exposure of dark-grown etiolated seedlings to light resulted in a rapid increase in the amount of Os-sigA mRNA in the shoot. These observations suggest that regulation of expression of the nuclear gene for this putative plastid RNA polymerase sigmafactor by light contributes to light-dependent transcriptional regulation of plastid genes.