Characterization of two plastid sigma factors, SigA1 and SigA2, that mainly function in matured chloroplasts in Nicotiana tabacum.

We have isolated and characterized two genes from Nicotiana tabacum, whose products function as putative sigma factors for plastid RNA polymerase. Since the amino acid sequence deduced from the DNA sequences of both genes showed highly similar to that of the SigA protein of Arabidopsis thaliana, we termed the corresponding genes sigA1 and sigA2, respectively. Transient expression assay using a green fluorescent protein (GFP) fusion construct indicated that the N-terminal region of the sigA2 gene product could function as a transit peptide for import into chloroplasts. The gel-blot analysis of RNAs revealed that the sum of the sigA1 and sigA2 transcripts fluctuated apparently with an endogenous rhythm after 12-h-light, 12-h-dark entrainment in photomixotrophically cultured tobacco cells. RT-PCR based northern analysis revealed that the sigA1 and sigA2 transcripts increased along with the cell growth in cultured cells, and were most abundant in mature leaves and shoot meristems with very young leaves in tobacco plants. Immunoblot analysis of the cell extracts of tobacco plants also supports this notion. These results suggest that the sigma factors encoded by sigA1 and sigA2 play a role in chloroplast development and regulation of gene expression in matured chloroplasts.