NUCLEAR ENCODING OF A PLASTID SIGMA-FACTOR IN RICE AND ITS TISSUE-DEPENDENT 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 nucleo tide sequence of the corresponding nuclear gene, named Os-sigA (O.sati va sigma A), predicts a polypeptide of 519 amino acids that contains a putative plastid-targeting sequence in its N-terminal region, The pre dicted mature protein shows extensive sequence homology to bacterial s igma 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, pro moter 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 etiola ted shoots of rice seedlings, Furthermore, exposure of dark-grown etio lated 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 polym erase sigma factor by light contributes to light-dependent transcripti onal regulation of plastid genes.