A VSV mutant synthesizes a large excess of functional mRNA but produces less viral protein than its wild-type parent

Abstract

A spontaneous small plaque mutant of VSV, S 2VSV, exhibits an unbalanced pattern of RNA synthesis in which transcription is increased and replication is decreased compared to the wild-type virus. A two- to threefold excess of messenger RNA (mRNA) is synthesized in the mutant-infected cells. Analysis of protein synthesis revealed that despite the production of a two- to threefold excess of mRNA, less than half of the amount of viral protein is synthesized in mutant as compared to wild type-infected cells. The study of S 2VSV-directed protein synthesis also showed a 6% decrease in the relative electrophoretic mobility of the S 2VSV NS protein and a 2% decrease in the relative migration of the mutant G protein. These alterations in mobility were not associated with a measurable change in the phosphorylation of NS or the glycosylation of G. Analysis of the structure and function of the S 2VSV mRNAs indicated that the reduction of viral protein synthesis in S 2VSV-infected cells is not due to the synthesis of defective mRNA. The mutant mRNAs are equivalent to wild-type VSV mRNAs with respect to size, length, and frequency of 3′-terminal poly(A) sequences, extent of capping of the 5′-termini, and efficiency of translation in vitro. The analysis of polysome distributions in wild-type VSV- and mutant-infected cells revealed however, that fewer S 2VSV mRNAs are associated with polyribosomes and these mRNAs are translated on smaller polysomes than wild-type VSV proteins. A decrease in the rate of initiation of translation on S,VSV mRNAs is indicated as the cause of the abnormal S 2VSV polysome profile.