An Arabidopsis sigma factor (SIG2)-dependent expression of plastid-encodedtRNAs in chloroplasts

A eubacteria-type RNA polymerase (PEP) plays crucial roles for chloroplast development in higher plants. The core subunits are encoded on plastid DNA (rpo genes) while the regulatory sigma factors are encoded on the nuclear D NA (SIG genes). However, the definite gene specificity of each sigma factor is unknown. We recently identified an Arabidopsis recessive pale-green mut ant abc1 in which TDNA is inserted in SIG2 (sigB). In this mutant, almost n ormal etioplasts were developed under dark conditions while the small chlor oplasts with poor thylakoid membranes and stacked lamellar were developed u nder light conditions. The sig2-1 mutant was deficient in accumulating enou gh photosynthetic and photosynthesis-related proteins as well as chlorophyl l. However, mRNAs of their structural genes were not significantly reduced. Further analyses revealed that several plastid-encoded tRNAs including trn E-UUC that has dual function for protein and ALA biosyntheses were drastica lly reduced in the sig2-1 mutant. In contrast, nucleus-encoded T7 phage-typ e RNA polymerase (TNEP)dependent gene transcripts were steadily accumulated in the mutant. These results indicate that progress of chloroplast develop ment requires SIG2-dependent expression of plastid genes, particularly some of the tRNA genes.