IDENTIFICATION OF THE GENE CODING FOR THE LARGEST SUBUNIT OF RNA-POLYMERASE-I(A) OF DROSOPHILA-MELANOGASTER

The gene coding for the largest subunit (RPA1) of RNA polymerase I (A) of Drosophila melanogaster (DmRPA1) was cloned and sequenced. The gen e is interrupted by seven small introns and the cDNA reveals an open r eading frame of 4932 nucleotides. The deduced polypeptide consists of 1644 amino acids with a calculated molecular weight of 185 kDa. Althou gh the protein sequence exhibits the specific pattern of conserved reg ions found in all RNA polymerase largest subunits characterized so far ! the overall sequence similarity among the RPA I subunits of differen t species is much lower than seen with the corresponding subunits of R NA polymerases II and III. Two highly divergent hydrophilic domains ch aracteristic for RPA1 separate the conserved blocks a and b in the N-t erminal region and blocks g and h in the C-terminal section, respectiv ely. In both cases the distance between the homologous blocks is enlar ged by about 70 amino acids relative to the largest subunits of RNA po lymerases II and III, and the corresponding subunit of the archaebacte rial enzyme. Compared with RPA1 sequences of lower eukaryotes, the C-t erminal hydrophilic domain in DmRPA1 is similar in length and acidity whereas the N-terminal domain is slightly shorter but retains the same basicity. The sequence insertions do not feature common motifs, sugge sting a role for them in the interaction of RNA polymerase I with prot eins required for the species-specific transcription of rDNA. The RPA1 subunits of Drosophila melanogaster and lower eukaryotes share an add itional Zn-binding motif at the N-terminus with archaebacterial and RP C1 subunits, testifying to the complex evolutionary relationships amon g the RNA polymerases.