The measles virus matrix gene and gene product defined by in vitro and in vivo expression.

Sequence analysis of full-length cDNA clones of the measles virus matrix gene revealed three possible open reading frames: M, X1, and X2. The M reading frame differed from the reported sequence by a single nucleotide corresponding to a conservative lysine to arginine amino acid substitution near the carboxy-terminus conserved among the M proteins of paramyxoviruses. The putative X reading frames contained no translational termination codon due to a frame-shift mutation. The protein-coding potential of these reading frames was examined by in vitro translation and DNA-mediated gene transfer into primate cells. The M reading frame produced a 38,000 Mr protein indistinguishable from the M protein in measles virus-infected cells. This protein was not phosphorylated nor processed post-translationally in vivo. The putative X1 and X2 reading frames could be translated into proteins when placed near the 5' terminus of the RNA. The resulting proteins were heterogeneous due to the lack of a termination codon. Translation from the putative X reading frames was adversely affected by an upstream AUG codon and these reading frames were unable to synthesize proteins in their normal 3' locations. At least 146 nucleotides of these 3'-untranslated sequences could be deleted without affecting the expression of M protein in vitro or in vivo. Thus despite the multiple open reading frames, the measles virus M gene is functionally monocistronic.