Characterization of the Masked Strain of Tobacco Mosaic Virus: Identification of the Region Responsible for Symptom Attenuation by Analysis of an Infectious cDNA Clone. Curits A. Holt. Department of Biology, Washington University, St. Louis, MO 63130. Richard A. J. Hodgson(1), Frank A. Coker(2), Roger N. Beachy(1), and Richard S. Nelson(2). (1)Department of Biology, Washington University, St. Louis, MO 63130, and (2)Plant Biology Division, Noble Foundation, Ardmore, OK 73402 U.S.A. MPMI 3:417-423. Accepted 31 July 1990. Copyright 1990 The American Phytopathological Society.

A strain of tobacco mosaic virus (TMV) that produces mild (attenuated) symptoms on tobacco plants has been molecularly cloned to identify the region of the genome responsible for symptom attenuation. A full-length cDNA clone whose transcripts produce the parental disease phenotype on both systemic and hypersensitive host plants has been constructed. This infectious clone was sequenced, and 55 base changes relative to the published sequence of common TMV (strain U1) were identified. These changes resulted in 12 amino acid alterations in the open reading frames encoding the 126/183-kDa and 30-kDa movement proteins; two of these changes were determined not to be responsible for the attenuated phenotype. Exchange of restriction fragments between the infectious mild strain cDNA and an infectious U1 strain cDNA indicated that the determinants involved in symptom attenuation reside in the open reading frame encoding the 126/183-kDa proteins of TMV; these proteins are involved in viral replication.