A splicing enhancer in the 3'-terminal c-H-ras exon influences mRNA abundance and transforming activity

Analysis of cDNA clones previously identified an optional intron in the 3'-untranslated region of the human H-ras gene. A possible correlation was observed between failure to remove this intron and overexpression of the gene, suggesting that splicing of the intron may require a specific titrable factor. The splicing signals at the end of the intron deviate from the consensus and may be inefficient, but we noticed that the adjacent exon downstream has a purine-rich region reminiscent of purine-rich splicing enhancers in other genes that stimulate the removal of weak, flanking introns. We show here that the purine-rich region of H-ras has splicing-enhancer activity in the homologous as well as a heterologous context. Interestingly, although the affected intron is outside the coding region, inversion or deletion of the enhancer reduced the transforming activity of oncogenic H-ras alleles severalfold. Experiments with corresponding cDNA constructs suggested that this is not a consequence of the altered structures of the mRNAs produced when the enhancer is inverted or deleted. Instead, we propose that the region controls an additional pre-mRNA processing event besides splicing of the terminal intron. Our work indicates that the purine-rich region may play an important role in the control of H-ras activity.