DEFECTIVE SPLICING INDUCED BY 4NQO IN THE HAMSTER HPRT GENE

The molecular analysis of mutations affecting mRNA processing may cont ribute to a better understanding of the splicing mechanism through the identification of genomic sequences necessary for the recognition of splice sites. In this paper we report the sequence analysis of 14 spli ce mutants induced by 4-nitroquinoline 1-oxide (4NQO) at the hamster h ypoxanthine-guanine-phosphoribosyltransferase (hprt) locus. We show th at mutations at the 3' acceptor splice site or at the first or fifth b ase of the 5' donor splice site are responsible for exon skipping. In addition, mutations in exon sequences also determine the skipping of o ne or more exons. Our data indicate that point mutations in intron reg ions at either side of an internal exon may induce the skipping of the same exon, supporting a model where the exon is the unit of early spl iceosome assembly. Furthermore, they suggest that the splicing of hprt mRNA precursors may proceed through a clustering of exons 2, 3 and 4 which are then spliced in a concerted way.