Spinach chloroplast rpoBC genes encode three subunits of the chloroplast RNA polymerase

Abstract

Sequence analysis of a 12,400 base-pair region of the spinach chloroplast genome indicates the presence of three genes encoding subunits of the chloroplast RNA polymerase. These genes are analogous to the rpoBC operon of Escherichia coli, with some significant differences. The first gene, termed rpoB, encodes a 121,000 M r homologue of the bacterial β subunit. The second and third genes, termed rpoC 1 and rpoC 2, encode 78,000 and 154,000 M r proteins homologous to the N and C-terminal portions, respectively, of the bacterial β′ subunit. RNA mapping analysis indicates that the three genes are cotranscribed, and that a single intron occurs in the rpoC 1 gene. No splicing occurs within the rpoC 2 gene or between rpoC 1 and rpoC 2. Furthermore, the data indicate the possibility of an alternative splice acceptor site for the rpoC 1 intron that would give rise to a 71,000 M r gene product. Thus, with the inclusion of the γ subunit encoded by rpoA at a separate locus, the chloroplast genome is predicted to encode four subunits (respectively called α, β, β′, β″) equivalent to the three subunits of the core enzyme of the E. coli RNA polymerase.