AU-rich intronic elements affect pre-mRNA 50 splice site selection in Drosophila melanogaster

cis-spliced nuclear pre-mRNA introns found in a variety of organisms, including Tetrahymena thermophila, Drosophila melanogaster, Caenorhabditis elegans, and plants, are significantly richer in adenosine and uridine residues than their flanking exons are. The functional significance of this intronic AU richness, however, has been demonstrated only in plant nuclei. In these nuclei, 5 ' and 3 ' splice sites are selected in part by their positions relative to AU-rich elements spread throughout the length of an intron. Because of this position-dependent selection scheme, a 5 ' splice site at the normal (+1) exon-intron boundary having only three contiguous consensus nucleotides can compete effectively with an enhanced exonic site (-57E) having nine consensus nucleotides and outcompete an enhanced site (+106E) embedded within the AU-rich intron. To determine whether transitions from AU-poor exonic sequences to AU-rich intronic sequences influence 5 ' splice site selection in other organisms, alleles of the pea rbcS3AI intron were expressed in Drosophila Schneider 2 cells, and their splicing patterns were compared with those in tobacco nuclei. We demonstrate that this heterologous transcript can be accurately spliced in transfected Drosophila nuclei and that a +1 G-to-A knockout mutation at the normal splice site activates the same three cryptic 5 ' splice sites as in tobacco. Enhancement of the exonic (-57) and intronic (+106) sites to consensus splice sites indicates that potential splice sites located in the upstream exon or at the 5 ' exon-intron boundary are preferred in Drosophila cells ove