Primary structure and functional aspects of the gene coding for the second-largest subunit of RNA polymerase III of Drosophila.

We have cloned and sequenced the gene coding for the second-largest subunit of RNA polymerase III of Drosophila melanogaster (DmRP135). The gene, interrupted by two introns of 62 and 59 bp, respectively, codes for an mRNA of 3.6 kb. As for other housekeeping genes transcription initiates at several sites (between positions -98 and -76) none of which is preceded by a clear TATA sequence. The deduced polypeptide consists of 1129 amino acids with an aggregate molecular weight of 128 kDa. The protein sequence features the same regions of similarity as observed for the corresponding subunits of RNA polymerase II of Drosophila and yeast and the Escherichia coli beta subunit. As in the second-largest subunit of RNA polymerase II there is a zinc-binding motif which is absent in the beta subunit of E. coli. Antibodies directed against a fusion protein expressing 164 amino acids of the DmRP135 polypeptide cross-react with the second-largest subunit of RNA polymerase III of yeast and generate a distinct banding pattern on Drosophila polytene chromosomes distinguishable from that obtained with anti-RNA polymerase II antibodies.