Human pre-mRNA cleavage factor Im is related to spliceosomal SR proteins and can be reconstituted in vitro from recombinant subunits.

Four polypeptides of 25, 59, 68, and 72 kDa copurify with the activity of human cleavage factor Im (CF Im) involved in pre-mRNA 3' end processing. We report here the cloning of the 25 and 68 kDa subunits and the reconstitution of functional CF Im25/68 from these two polypeptides. Several lines of evidence indicate that CF Im exists in at least two different forms. The 68 kDa polypeptide has a domain organization reminiscent of spliceosomal SR proteins. Analysis of the kinetics of the cleavage reaction indicates that interaction of CF Im with the RNA is one of the earliest steps in the assembly of the 3' end processing complex and facilitates the recruitment of other processing factors.