A tobacco mosaic virus mutant with non-functional coat protein and its revertant: Relationship to the virus assembly process

Abstract

A nitrous acid-induced coat protein-defective mutant (PM6) derived from the wild-type strain (U1) of tobacco mosaic virus (TMV) and its coat protein-functional revertant (PM6R) have been isolated. PM6 is a virus assembly mutant in which the coat protein cannot encapsidate viral RNA. The coat protein of PM6 forms unusual wheel-like structures consisting of aggregated disks with a helical conformation. PM6 protein has two amino acid substitutions when compared to U1-TMV, an Ala to Thr exchange at position 105 and an Asp to Gly exchange, probably at position 88. PM6R has the Ala to Thr exchange found in PM6 at position 105, but the Asp to Gly exchange observed in PM6 has apparently reverted to the native sequence found in U1-TMV. A high frequency of irregular protein rods is observed in electron micrographs of PM6R, suggesting that the conformation of PM6R coat protein, although functional, is altered from that of the wild-type strain.