THE G-RICH AUXILIARY DOWNSTREAM ELEMENT HAS DISTINCT SEQUENCE AND POSITION REQUIREMENTS AND MEDIATES EFFICIENT 3' END PRE-MESSENGER-RNA PROCESSING THROUGH A TRANS-ACTING FACTOR

A downstream G-rich sequence (GRS), GGGGGAGGUGUGGG, has been previousl y shown to influence the efficiency of 3' end processing of the SV40 l ate polyadenylation signal, We have now defined several important para meters for GRS-mediated polyadenylation. The ability of the GRS to inf luence 3' end processing efficiency was sensitive to individual and mu ltiple point mutations within the element, as well as the position of the element in the downstream region, Competition analysis indicated t hat the GRS functioned through a titratable trans-acting factor, The G RS-specific DSEF-1 protein was found to be bound to the same populatio n of RNAs as the 64 kDa protein of the general polyadenylation factor CstF, indicating that DSEF-1 is associated with RNA substrates undergo ing 3' end processing, Furthermore, an association was obtained betwee n the relative strength of DSEF-1 protein binding to GRS variants and the relative ability of the GRS variants to mediate efficient cleavage in vitro, Finally, mutations in the GRS affected the efficiency of cr oss-linking of the 64 kDa protein of CstF These data define a novel cl ass of auxiliary downstream element and suggest an important role for DSEF-1 in 3' end processing.