A COMBINATORIAL ROLE FOR EXON, INTRON AND SPLICE-SITE SEQUENCES IN SPLICING IN MAIZE

Plant introns are typically AU-rich or U-rich, and this feature has be en shown to be important for splicing. In maize, however, about 20% of the introns exceed 50% GC, and most of them are efficiently spliced. A series of constructs has been designed to analyze the cis requiremen ts for splicing of the GC-rich Bz2 maize intron and two other GC-rich intron derivatives. By manipulating exon, intron and splice site seque nces it is shown that exons can play an important role in intron defin ition: changes in exon sequences can increase splicing efficiency of a GC-rich intron from 17% to 86%. The relative difference, or base comp ositional contrast, in GC and U content between exon and intron sequen ces in the vicinity of splice sites, rather than the absolute base-con tent of the intron or exons, correlates with splicing efficiency. It i s also shown that GC-rich intron constructs that are poorly spliced ca n be partially rescued by an improved 3' splice site.