Phenotypic revertants of temperature-sensitive M protein mutants of XXXX virus: sequence analysis and functional characterization.

@entity1522 @entity2036 @entity535 @entity8310 @entity377
Twenty-five spontaneous temperature-stable revertants of four different temperature-sensitive (ts) M protein mutants (complementation group III: tsG31, tsG33, tsO23, and tsO89) were sequenced and tested for their ability to inhibit @entity8310 virus RNA polymerase activity in vitro. Consensus sequences of the coding region of each M protein gene were determined, using total viral RNA as template. Fifteen different sequences were found among the 25 revertants; 14 differed from their ts parent by a single @entity535 (one @entity377 ), and 1 differed by two @entity1522 (two @entity2036 ). @entity1522 were altered in various positions between residues 64 and 215, representing over 60% of the polypeptide chain. Resequencing of the Glasgow and Orsay wild types and the four ts mutants confirmed previously published differences (Y. Gopalakrishana and J. Lenard, J. Virol., 56:655-659, 1985), and one or two additional differences were found in each. The relative charges of the revertant M proteins, as determined by nonequilibrium pH gradient electrophoresis, were consistent with the deduced sequences in every case. The ability of each revertant M protein to inhibit the RNA polymerase activity of nucleocapsids prepared from its parent ts mutant was also tested. Only 13 of the 25 revertants had M protein with high (wild type-like) polymerase-inhibiting activity, while 5 had low (ts-like) activity, and 7 had intermediate activity, demonstrating that this property is not an essential concomitant of the temperature-stable phenotype. It is concluded that the high reversion frequency observed for these mutants arises from a very high incidence of pseudoreversion, i.e., many different molecular changes can repair the ts phenotype.
