Mutational analysis of the movement protein of odontoglossum ringspot virus to identify a host-range determinant.

Tobacco mosaic virus (TMV) which contains the movement protein (MP) of odontoglossum ringspot tobamovirus (ORSV) in place of the TMV MP systemically infects orchids but causes local infection in tobacco unless the carboxy-terminal 48 amino acids of the MP are deleted (C. A. Holt, C. A. Fenczik, S. J. Casper, and R. N. Beachy; Virology, in press, 1995). Frameshift mutations were created within the 3' ends of the MP gene that led to truncations of the ORSV MP by 11, 19, 28, 37, and 48 amino acids; each of the mutant MP genes was inserted into the cloned cDNA of TMV in place of the TMV MP and infectious transcripts were produced. Virus containing mutant MPs were used to infect vanilla orchids, a systemic host of ORSV, and tobacco plants. Removal of 11 amino acids from the ORSV MP prevented spread of the chimeric virus in orchids while restoring the ability to cause a systemic infection on tobacco. Further deletions of the MP affected the size of virus-induced necrotic local lesions on tobacco cv. Xanthi NN and the systemic spread and accumulation of virus in cv. Xanthi nn, a systemic host of TMV. However, each virus replicated to equivalent levels in protoplasts. A mechanism by which the ORSV MP limits the spread of the chimeric virus is proposed.