Further implications for the evolutionary relationships between tripartite plant viruses based on cucumber mosaic virus RNA 3.

The nucleotide sequence of the RNA 3 of the Q-strain of cucumber mosaic virus (Q-CMV) has been reinvestigated and supporting partial amino acid sequence data obtained for the coat protein. Corrections to the previously published sequence of RNA 3 [A. R. Gould and R. H. Symons (1982) Eur. J. Biochem. 126, 217-226] result in changes to the size and composition of the putative 3a and coat proteins. Analysis of the nucleotide sequence revealed a 14-nucleotide sequence present in the intercistronic regions of the RNA 3 molecules of both Q-CMV and brome mosaic virus (BMV). This sequence, which is closely related to sequences previously detected in the 5'-untranslated region of Q-CMV and BMV RNAs 1 and 2 [M. A. Rezaian, R. H. V. Williams, and R. H. Symons (1985) Eur. J. Biochem. 150, 331-339], may be important in the control of RNA synthesis. Computer-assisted comparisons indicate an ancestral relationship between the 3a proteins of CMV, BMV, and alfalfa mosaic virus (AMV) and between the coat proteins of CMV and BMV. These comparisons significantly extend previous observations regarding the close evolutionary relationships within the plant tripartite virus group.