A temperature-sensitive lethal mutation in Saccharomyces cerevisiae, prp20-1, causes defects in several different steps in mRNA metabolism, including mRNA 3'-end formation, transcription initiation, and mRNA transport. Previous work has demonstrated that prp20 mutants are defective in actin pre-mRNA splicing. PRP20 is related, both in structure and function, to the RCC1 gene of mammals and the PIM1 gene of Schizosaccharomyces pombe, both of which appear to regulate entry into mitosis and chromosome condensation. In this report we demonstrate that, after a shift of prp20 mutants to the restrictive temperature, transcripts of several genes (CUP1, CYH2, and GAL10) are produced that extend 1-10 kb beyond their normal polyadenylation sites. The failure in 3'-end formation occurs within 1-2 min of the temperature shift. Transcription initiation also is disrupted, in that initiation sites upstream of the normal cap site are used. mRNA transport from nucleus to cytoplasm also is perturbed: In situ hybridization using an oligo(dT) probe demonstrates accumulation of poly(A) in the nucleus, consistent with the accumulation of longer bulk poly(A) (up to approximately 90-100 nucleotides) and with a failure to transport newly synthesized RNA to the cytoplasm. We demonstrate that prp20 and rna1 mutants are very similar, if not identical, with respect to each of these biochemical phenotypes. In light of the putative role of PRP20 in mitotic control, our results suggest a common step in that process and multiple steps in mRNA synthesis and maturation. We speculate that the perturbations in mRNA processing are the result of effects on the chromatin-nascent RNP-transcription complex or misregulation of a cell cycle component that modifies multiple mRNA-processing activities.