EVIDENCE THAT SIGMA-FACTORS ARE COMPONENTS OF CHLOROPLAST RNA-POLYMERASE

Plastid genes are transcribed by DNA-dependent RNA polymerase(s), whic h have been incompletely characterized and have been examined in a lim ited number of species. Plastid genomes contain rpoA, rpoB, rpoC1, and rpoC2 coding for alpha, beta, beta', and beta'' RNA polymerase subuni ts that are homologous to the alpha, beta, and beta' subunits that con stitute the core moiety of RNA polymerase in bacteria. However, genes with homology to sigma subunits in bacteria have not been found in pla stid genomes. An antibody directed against the principal sigma subunit of RNA polymerase from the cyanobacterium Anabaena sp. PCC 7120 was u sed to probe western blots of purified chloroplast RNA polymerase from maize, rice, Chlamydomonas reinhardtii, and Cyanidium caldarium. Chlo roplast RNA polymerase from maize and rice contained an immunoreactive 64-kD protein. Chloroplast RNA polymerase from C. reinhardtii contain ed immunoreactive 100- and 82-kD proteins, and chloroplast RNA polymer ase from C. caldarium contained an immunoreactive 32-kD protein. The e lution profile of enzyme activity of both algal chloroplast RNA polyme rases coeluted from DEAF with the respective immunoreactive proteins, indicating that they are components of the enzyme. These results provi de immunological evidence for sigma-like factors in chloroplast RNA po lymerase in higher plants and algae.