RNA-POLYMERASE SUBUNITS ENCODED BY THE PLASTID RPO GENES ARE NOT SHARED WITH THE NUCLEUS-ENCODED PLASTID ENZYME

Plastid genes in photosynthetic higher plants are transcribed by at le ast two RNA polymerases. The plastid rpoA, rpoB, rpoC1, and rpoC1, gen es encode subunits of the plastid-encoded plastid RNA polymerase (PEP) , an Escherichia coli-like core enzyme. The second enzyme is referred to as the nucleus-encoded plastid RNA polymerase (NEP), since its subu nits are assumed to be encoded in the nucleus. Promoters for NEP have been previously characterized in tobacco plants lacking PEP due to tar geted deletion of rpoB (encoding the beta-subunit) from the plastid ge nome. To determine if NEP and PEP share any essential subunits, the rp oA, rpoC1, and rpoC2 genes encoding the PEP alpha-, beta'-, and beta ' '-subunits were removed by targeted gene deletion from the plastid gen ome. We report here that deletion of each of these genes yielded photo synthetically defective plants that lack PEP activity while maintainin g transcription specificity from NEP promoters. Therefore, rpoA, rpoB, rpoC1, and rpoC2 encode PEP subunits that are not essential component s of the NEP transcription machinery. Furthermore, our data indicate t hat no functional copy of rpoA, rpoB, rpoC1, or rpoC2 that could compl ement the deleted plastid rpo genes exists outside the plastids.