Abstract
The subunits of yeast RNA polymerases A(I) and B(II) were characterized using several techniques. The present studies demonstrate that the A and B enzymes possess three subunits, which are indistinguishable on the basis of molecular weight, isoelectric point, and fingerprint pattern. The three common subunits belong to the small molecular weight components of the enzymes. By polyacrylamide gel electrophoresis with sodium dodecyl sulfate they migrate with apparent molecular weights of 27,000, 23,000, and 14,500, respectively. A two-dimensional subunit mapping technique on polyacrylamide gel was used to separate the subunits according to isoelectric point and molecular weight. The common polypeptides co-migrated on three spots corresponding to isoelectric points of 9.2 (27,000), 4.5 (23,000), and 4.6 (14,500). The fingerprints of the 35S-labeled tryptic peptides of the presumptive common subunits were found to be essentially identical. Finally, the presence of common subunits was supported by the fact that antibodies against pure RNA polymerase A cross-react with and inhibit RNA polymerase B. Except for the common subunits, it is likely that RNA polymerases A and B are primarily made of distinct gene products for the following reasons. A total of 13 polypeptide chains are present in enzyme A, whereas 10 polypeptides are found in enzyme B. The molecular weight, isoelectric point, and sulfur content of the majority of these polypeptide chains are different in the two enzymes. No similarity was found in the 35S-peptide fingerprint from a number of A and B subunits of slightly different molecular weight. Finally, antibodies against the largest subunit from RNA polymerase A do not cross-react with or inhibit RNA polymerase B. The data are discussed in terms of structural organization of eukaryotic RNA polymerases.
Highlights
The subunits of yeast RNA polymerases A(1) and B(I1) were characterized using several techniques
The present studies demonstrate that the A and B enzymes possess three subunits, which are indistinguishable on the basis of molecular weight, isoelectric point, and fingerprint pattern
The possible existence of a pool of small subunits common to the different classes of nuclear enzymes was suggested by the finding of polypeptide chains having identical molecular weight in mammalian enzymes [4, 6] as well as in yeast RNA polymerases
Summary
The subunits of yeast RNA polymerases A(1) and B(I1) were characterized using several techniques. All the nuclear enzymes purified so far from mammalian or lower eukaryotic cells present a striking structural similarity. They are isolated as large multiprotein complexes containing two high molecular weight subunits and several smaller polypeptide chains [5,6,7,8,9,10]. The possible existence of a pool of small subunits common to the different classes of nuclear enzymes was suggested by the finding of polypeptide chains having identical molecular weight in mammalian enzymes [4, 6] as well as in yeast RNA polymerases. Immunological studies revealed that at least A and B enzymes shared common antigenic determinants [12, 13]
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