Both one-and two-dimensional pe tide mapping techniques have been used to compare the 180-kilodalton subunits from maize chloroplast and type II nuclear DNAdependent RNA polymerases (nucleosidetriphosphate:RNA nucleotidyltransferase, EC 2.7.7.6). Despite their similar molecular mass, these 180-kilodalton subunits are not the same. Also, two-dimensional tryptic maps of the 160-, 43-, and 28-kilodalton polypeptides from maize type II nuclear RNA polymerase and of the 140-, 42-, and 27-kilodalton polypeptides from maize chloroplast RNA polymerase show that each of these six polypeptides is unique. Cells of eukaryotic green plants contain at least three separate genomes. DNA is found together with DNA-dependent RNA polymerases (nucleosidetriphosphate:RNA nucleotidyltransferase, EC 2.7.7.6) in their chloroplasts and mitochondria as well as in their nuclei (1-4). The maintenance and development of each organelle, as well as of the cell as a whole, require the participation of all three genomes. For example, ribosomes and other multimeric components of organelles are composed of organelle-and nuclear-coded elements (4). So, not only does RNA synthesis have to be regulated within each organelle, but also gene expression in such multigenomic cells must be coordinated and integrated. Intracellular integration of transcription could be effected through functional ties between nuclear and organellar RNA polymerases. Chloroplast and type II nuclear RNA polymerases have been prepared from subcellular fractions of maize leaf homogenates (5-14). Sodium dodecyl sulfate/polyacrylamide gel electrophoresis of these two maize RNA polymerases has shown that each enzyme contains two large subunits which are present at all stages of purification. The molecular masses of these polypeptides are 180 and 160 kilodaltons (kDal) for the type II nuclear RNA polymerase (12, 14, 15) and 180 and 140 kDal for the chloroplast enzyme (11). Moreover, when a sample containing both the plastid and nuclear enzymes is electrophoresed through a single denaturing gel, the 180-kDal subunits of the two RNA polymerases are not resolved (12). Intracellular-intergenomic integration of transcription via sharing of a common subunit by chloroplast and type II nuclear RNA polymerases seems an appealing possibility.In this paper we compare, by one-and two-dimensional mapping, proteolytic fragments of the 180-kDal subunits as well as other pairs of polypeptides from maize chloroplast and type II nuclear RNA polymerases. None of the pairs of subunits of the two enzymes resembles one another.
MATERIALS AND METHODSPlants. Maize seeds (Zea mays, FR9c ms X FR37, Illinois Foundation Seeds) were soaked in water, then sown in moist vermiculite, and grown in a greenhouse for 7-9 days with supplemental light.RNA Polymerases and Subunits. Chloroplast RNA polymerase was solubilized from isolated maize chloroplasts (7) and was purified by glycerol gradient centrifugation and phosphocellulose chromatography (11, 13). Type II nuclear RNA polymerase was purified as describe...