FUNCTIONAL-ANALYSIS OF AN INTRON 3'-SPLICE-SITE IN CAENORHABDITIS-ELEGANS

Caenorhabditis elegans introns typically lack both branch point and po lypyrimidine tract consensus sequences utilized in other organisms for intron recognition. However, most introns have an unusually long, hig hly conserved consensus, UUUCAG/R, at the 3' splice site. This site ca n be recognized even when the -1G is changed to A. To determine how th e 3' splice site is defined, we tested mutations in the Sequence UUUCA A/A at the 3' splice site of the first intron of a test gene in vivo. Replacement of individual U's with A's or C's compromised splicing. Wh en the sequence UUUCAA/AAG was tested, splicing occurred following bot h the -1A and the +3G, indicating that both UUUC and the AG contain 3' Splice site information. When the sequence UUUCAA/AAA was tested, all splicing occurred following the -1A, suggesting that the UUUC contain s sufficient information in the absence of an AG to specify the locati on of the splice site. In support of this idea, when point mutations w ere introduced into the UUUC, unspliced RNAs accumulated. Surprisingly , RNAs containing the mutant intron often contained the second, nonmut ated intron as well, suggesting that interference with splicing of one intron can interfere with splicing of a second intron in the same pre -mRNA. The majority of these unspliced RNAs were degraded by the syste m responsible for degradation of transcripts containing nonsense mutat ions (smg), even though the intron contained no nonsense codon.