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Ribonuclease T1 (RNase T1) cleaves the phosphodiester bond of RNA specifically
at the 3'-end of guanosine. 2'-guanosinemonophosphate (2'-GMP) acts as inhibitor
for this reaction and was cocrystallized with RNase T1. X-Ray analysis provided
insight in the geometry of the active site and in the parts of the enzyme
involved in the recognition of guanosine. RNase T1 is globular in shape and
consists of a 4.5 turns alpha-helix lying "below" a four-stranded antiparallel
beta-sheet containing recognition center as well as active site. The latter is
indicated by the position of phosphate and sugar residues of 2'-GMP and shows
that Glu58, His92 and Arg77 are active in phosphodiester hydrolysis. Guanine is
recognized by a stretch of protein from Tyr42 to Tyr45. Residues involved in
recognition are peptide NH and C = O, guanine O6 and N1H which form hydrogen
bonds and a stacking interaction of Tyr45 on guanine. Although, on a theoretical
basis, many specific amino acid-guanine interactions are possible, none is
employed in the RNase T1.guanine recognition.
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