THE FUNCTION OF MULTISITE SPLICING ENHANCERS

Splicing enhancers are RNA sequences consisting of one or more binding sites (enhancer elements) for specific serine/arginine (SR)-rich prot eins. When associated with these elements, SR proteins activate splici ng by recruiting the splicing machinery to the adjacent intron through protein-protein interactions. Here, we show that the rate and efficie ncy of splicing increases linearly, rather than synergistically, as th e number of identical or nonidentical enhancer elements present on pre -mRNA is increased. We conclude that only one splicing enhancer comple x at a time is capable of interacting with the constitutive splicing m achinery. Thus, the function of multisite enhancer elements is to incr ease the probability of an interaction between the enhancer complex an d the splicing machinery rather than to promote functional synergy.