Caenorhabditis elegans mRNAs that encode a protein similar to ADARs derive from an operon containing six genes

The Caenorhabditis elegans T20H4.4 open reading frame (GenBank accession no. U00037) predicted by Genefinder encodes a 367 amino acid protein that is 32-35% identical to the C-terminal domain of adenosine deaminases that act on RNA. We show that T20H4.4 cDNAs (GenBank accession no. AF051275) encode a larger 495 amino acid protein that is extended at its N-terminus to include a single double-stranded RNA-binding motif, and that T20H4.4 occupies the second position in a six-gene operon (5'-T20H4.5, T20H4.4, R151.8A, R151.8B, R151.7, R151.6-3'). Ten different spliced-leader (SL) sequences were found attached to T20H4.4 mRNAs, including SL1, SL2 and eight SL2-like leaders that include two new variants. Characterization of cDNAs derived from all six genes confirmed the essential features of C.elegans operons: intercistronic distances in the range of 104-257 nt between the upstream polyadenylation sites and the downstream trans -splice sites; SL2, or SL2-like leaders, attached to the downstream mRNAs. Polycistronic mRNA fragments revealed a 5'-untranslated region (5'-UTR) >705 nt. The 5'-UTR is removed in mature mRNAs from the first gene (T20H4.5) and replaced primarily by SL1, and to a lesser extent by SL2. Our study provides new information regarding operons and how they are processed.