We describe the cloning and characterization of a human homolog of the yeast 
transcription/RNA-processing factor Ssu72, following a yeast two-hybrid screen 
for pRb-binding factors in the prostate gland. Interaction between hSsu72 and 
pRb was observed in transfected mammalian cells and involved multiple domains in 
pRb; however, so far, mutual effects of these two factors could not be 
demonstrated. Like the yeast counterpart, mammalian Ssu72 associates with TFIIB 
and the yeast cleavage/polyadenylation factor Pta1, and exhibits intrinsic 
phosphatase activity. Mammals contain a single ssu72 gene and a few pseudogenes. 
During mouse embryogenesis, ssu72 was highly expressed in the nervous system and 
intestine; high expression in the nervous system persisted in adult mice and was 
also readily observed in multiple human tumor cell lines. Both endogenous and 
ectopically expressed mammalian Ssu72 proteins resided primarily in the 
cytoplasm and only partly in the nucleus. Interestingly, fusion to a strong 
nuclear localization signal conferred nuclear localization only in a fraction of 
transfected cells, suggesting active tethering in the cytoplasm. Suppression of 
ssu72 expression in mammalian cells by siRNA did not reduce 
proliferation/survival, and its over-expression did not affect transcription of 
candidate genes in transient reporter assays. Despite high conservation, hssu72 
was unable to rescue an ssu72 lethal mutation in yeast. Together, our results 
highlight conserved and mammalian specific characteristics of mammalian ssu72.