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Characterization of a protein from Rice tungro spherical virus with serine proteinase-like activity

John Innes Centre, Department of Metabolic Biology¹ and Department of Disease and Stress Biology², Norwich Research Park, Norwich NR4 7UH, UK

Author for correspondence: Roger Hull. Fax +44 1603 450045. e-mail roger.hull{at}bbsrc.ac.uk

The RNA genome of Rice tungro spherical virus (RTSV) is predicted to be expressed as a large polyprotein precursor (Shen et al., Virology 193, 621–630, 1993 ). The polyprotein is processed by at least one virus-encoded protease located adjacent to the C-terminal putative RNA polymerase which shows sequence similarity to viral serine-like proteases. The catalytic activity of this protease was explored using in vitro transcription/translation systems. Besides acting in cis, the protease had activity in trans on precursors containing regions of the 3' half of the polyprotein but did not process a substrate consisting of a precursor of the coat proteins. The substitution mutation of Asp²⁷³⁵ of the RTSV polyprotein had no effect on proteolysis; however, His²⁶⁸⁰, Glu²⁷¹⁷, Cys²⁸¹¹ and His²⁸³⁰ proved to be essential for catalytic activity and could constitute the catalytic centre and/or substrate-binding pocket of the RTSV 3C-like protease.