TRANSCRIPTION OF A NEMATODE U1 SMALL NUCLEAR-RNA IN-VITRO - 3'-END FORMATION REQUIRES CIS-ACTING ELEMENTS WITHIN THE CODING SEQUENCE

We have used block-substitution mutagenesis and in vitro transcription assays to identify cis-acting DNA sequence elements important for ini tiation and 3'-end formation of a U1 small nuclear RNA (snRNA) in the parasitic nematode Ascaris lumbricoides. Efficient initiation of synth esis by RNA polymerase II requires a compact element centered similar to 50 base pairs upstream of the transcriptional start site. Surprisin gly, 3'-end formation of U1 snRNAs synthesized in vitro is solely depe ndent upon elements within the U1 coding sequence. In all other system s studied thus far, 3'-end formation of U snRNAs requires signals pres ent in the 3'-flanking region. We also show that sequence elements tha t direct 3'-end formation of the A. lumbricoides trans-spliced leader RNA function when RNA synthesis is initiated from the U1 promoter. The se results indicate that 3'-end formation of U snRNAs in nematodes is mechanistically distinct from the analogous process in higher eukaryot es.