S1_PNPase: Polynucleotide phosphorylase (PNPase), ), S1-like RNA-binding domain. PNPase is a polyribonucleotide nucleotidyl transferase that degrades mRNA. It is a trimeric multidomain protein. The C-terminus contains the S1 domain which binds ssRNA. This family is classified based on the S1 domain. PNPase nonspecifically removes the 3' nucleotides from mRNA, but is stalled by double-stranded RNA structures such as a stem-loop. Evidence shows that a minimum of 7-10 unpaired nucleotides at the 3' end, is required for PNPase degradation. It is suggested that PNPase also dephosphorylates the RNA 5' end. This additional activity may regulate the 5'-dependent activity of RNaseE in vivo.
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