Unexpectedly, however, Ssu72 inactivation increased the efficiency of 39-end formation of non-polyadenylated replicationdependent histone mRNA. Furthermore, Ssu72 depletion caused a significant increase in both Ser5 and Ser7 phosphorylation on all genes in which 39-end processing was affected. These results suggest that vertebrate Ssu72 plays positive roles in 39-end formation of snRNAs and polyadenylated mRNAs, but a negative role in 39-end formation of histone mRNAs, by dephosphorylating both Ser5P and Ser7P of the RNA Pol II CTD. In this study, we established Ssu72 conditional-knockout chicken DT40 cell lines and used them to investigate the functions of Ssu72 in gene expression at the cellular level. Our results demonstrated that chicken Ssu72, similar to its yeast ortholog, is essential for cell viability and efficient 39-end formation of at least some snRNAs, as well as polyadenylated mRNAs. Unexpectedly, however, inactivation of Ssu72 caused a rapid and marked decrease in the levels of unprocessed replication-dependent histone mRNAs, indicating that chicken Ssu72 normally suppresses the stem-loop�Ctype 39-end formation of histone mRNA. Furthermore, Ssu72 inactivation led to a marked increase in both Ser5 and Ser7 phosphorylation on all genes in which 39-end formation was affected. This is the first demonstration that Ssu72 functions as a Ser5P/Ser7P phosphatase in vertebrate cells. Together, our findings suggest that vertebrate Ssu72 is involved in various ways in 39-end formation of Pol II transcripts, and that its function is Labetalol hydrochloride mediated by dephosphorylation of CTD at both Ser5 and Ser7. At present, we do not know which aspects of Ssu72 functions are required for cell viability. A recent report showed that Creatinine replacement of the Ser7 residue with phospho-mimetic glutamate in all CTD heptapeptide repeats is lethal, both in budding yeast and in mammals, suggesting that persistent elevation of the Ser7P level dramatically decreases cell viability. As shown in this study, Ssu72 depletion results in dramatic elevation of Ser7P levels on several Pol II genes, even though depletion exerts only a modest effect on overall levels of Ser7P.