Four factors are required for 3'-end cleavage of pre-mRNAs.

We reported previously that authentic polyadenylation of pre-mRNAs in vitro requires at least two factors: a cleavage/specificity factor (CSF) and a fraction containing nonspecific poly(A) polymerase activity. To study the molecular mechanisms underlying 3' cleavage of pre-mRNAs, we fractionated CSF further and show that it consists of four separable subunits. One of these, called specificity factor (SF; Mr, approximately 290,000), is required for both specific cleavage and for specific polyadenylation and thus appears responsible for the specificity of the reaction. Although SF has not been purified to homogeneity, several lines of evidence suggest that it may not contain an essential RNA component. Two other factors, designated cleavage factors I (CFI; Mr, approximately 130,000) and II (CFII; Mr, approximately 110,000), are sufficient to reconstitute accurate cleavage when mixed with SF. A fourth factor, termed cleavage stimulation factor (CstF; Mr, approximately 200,000), enhances cleavage efficiency significantly when added to a mixture of the three other factors. CFI, CFII, and CstF do not contain RNA components, nor do they affect specific polyadenylation in the absence of cleavage. Although these four factors are necessary and sufficient to reconstitute efficient cleavage of one pre-RNA tested, poly(A) polymerase is also required to cleave several others. A model suggesting how these factors interact with the pre-mRNA and with each other is discussed.