In a search for proteins associated with Rna15p in processing the 3 ends of messenger RNAs, we have looked for suppressors that correct, even partially, the thermosensitive growth defect of therna15-2 mutant. Mutations in a single locus that we namedSSM5, were able to suppress both the thermosensitivity of cell growth and the mRNA 3 processing defect associated with therna15-2 mutation, but only slightly alleviated the thermosensitive growth defect of anrna14-1 mutant. Thessm5-1 mutant is sensitive to hydroxyurea at 37° C, a drug that inhibits DNA synthesis. By screening for complementation of the hydroxyurea-sensitive phenotype we cloned the corresponding wild-type gene and found that it corresponds to the essential geneSTS1 (also namedDBF8). Sts1p has an apparent molecular weight of 30 kDa and was confirmed to be a cytosolic protein by immunofluorescence analysis. Western blot analysis indicates that the thermosensitive mutant strainsrna15-2, rna14-1 andpap1-1 present a very low level of the Rna15p at 37° C. Thessm5-1 mutation restores the level of Rna15p in therna15-2 ssm5-1 double mutant. Use of the two-hybrid system suggests that Sts1p does not interact directly with Rna15p, but may be active as a homodimer. The present data suggest that Sts1p may play a role in the transport of Rna15p from the cytoplasm to the nucleus.