THE GENE UPSTREAM OF DMRP128 CODES FOR A NOVEL GTP-BINDING PROTEIN OF DROSOPHILA-MELANOGASTER

Upstream of the gene coding for the second-largest subunit of RNA polymerase III (DmRP128) we have found another gene (128up), which is transcribed in the same direction as the RNA polymerase gene. The intergenic distance between the 3' end of 128up mRNA and the 5' end of DmRP128 mRNA is only about 100 bp. Transcripts of 128up are present at a much higher level than DmRP128 RNA in Drosophila Schneider 2 cells, embryos, and adult flies. Two transcription start points, seven nucleotides apart, are found for 128up compared to multiple scattered starts for DmRP128. Sequence analysis of 128up cDNA reveals that the gene codes for a 41 kDa protein with homology to GTP-binding proteins and matching four of the structural sequence motifs characteristic of the superfamily of GTPases. Bacterially expressed 128up protein fused to maltose-binding protein specifically binds GTP. Sequences closely related to the 128up protein are found in species as distant as Halobacterium, yeast or mouse; the murine protein is 80% identical to 128up. This evolutionary conservation is indicative of an important, but as yet unknown, physiological role. In accordance with the sequence conservation, antibodies against 128up specifically cross-react with mouse 3T3 cells and human Hep2 cells where the subcellular localization of the protein is predominantly perinuclear. We propose that 128up is a member of a novel class of GTP-binding proteins.