H/ACA ribonucleoprotein particle (RNP),the most complicated RNA pseudouridylase,uses H/ACA guide RNA for substrate capture and four proteins (Cbf5,Nop10,L7Ae,and Gar1) for pseudouridylation.Gar1 has been shown not only to facilitate product release but also,more importantly,enhances the catalytic rate1.The molecular mechanism of the catalytic promotion by Gar1 remains unclear.Using FCS and anisotropy measurements,we have found that although Gar1 greatly promotes the catalytic reaction,it does not affect the activation energy of pseudouridylation by H/ACA RNP.