THE CAP AND THE 3'-SPLICE-SITE SIMILARLY AFFECT POLYADENYLATION EFFICIENCY

The 5' cap of a mammalian pre-mRNA has been shown to interact with spl icing components at the adjacent 5' splice site for processing of the first exon and the removal of the first intron (E. Izaurralde, J. Le,v is, C. McGuigan, M. Jankowska, E. Darzynkiewicz, and I. W. Mattaj, Cel l 78:657-668, 1994). Likewise, it has been shown that processing of th e last exon arid removal of the last intron involve interaction betwee n splicing components at the 3' splice site and the polyadenylation co mplex at the polyadenylation signal. (M. Niwa, S. D. Rose, and S. M. B erget, Genes Dev. 4:1552-1559, 1990; M. Niwa and S. M. Berget, Genes D ev. 5:2086-2095, 1991). These findings suggest that the cap provides a function in first exon processing which is similar to,the function of the 3' splice site in last exon processing. To determine whether caps and 3' splice sites function similarly, we compared the effects of th e cap arid the 3' splice site on the in vitro utilization of the simia n virus 40 late polyadenylation signal. We show that the presence of a m(7)GpppG cap, but not a cap analog, can positively affect the effici ency of polyadenylation of a polyadenylation-only substrate. Cap analo gs do not stimulate polyadenylation because they fail to bind titratab le cap-binding factors. The failure of cap analogs to stimulate polyad enylation can be overcome if a 3' splice site is present upstream of t he polyadenylation signal. These data indicate that factors interactin g with the cap or the 3' splice site function Similarly to affect poly adenylation efficiency and complete exon processing. We also find that a 5' splice site directly upstream of the polyadenylation signal, alo ng with a m(7)GpppG cap, is inhibitory to polyadenylation. This findin g suggests that the interaction between the cap-binding complexes and splicing components at the 5' splice site may form a complex which is inhibitory to further processing if splicing of an adjacent intron is not achieved.