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Abstract:
Ssu72 is an essential and highly conserved protein
involved in mRNA transcription and 3-end processing.
The biochemical function of Ssu72 was so far unknown.
We report here evidence that Ssu72 is a phosphatase
that resembles protein tyrosine phosphatases
(PTPases). First, recombinant Ssu72 cleaves the phosphotyrosine
analogue p-nitrophenylphosphate, and this
catalytic activity is impaired by PTPase-inhibiting
agents. Second, the Ssu72 sequence contains the CX5R
signature motif of PTPases; mutation of the catalytic
cysteine in this motif abolishes Ssu72 activity in vitro
and has been shown to confer lethality in vivo. Third,
secondary structure prediction and site-directed mutagenesis
predict that Ssu72 adopts the fold of PTPases
of the low molecular weight family. Distinguishing features,
such as a short “aspartate loop” at the active site,
suggest however that Ssu72 is the founding member of a
new phosphatase subfamily. The novel Ssu72 activity
may regulate coupling events during mRNA biogenesis.