Evidence for a role of methylation in the processing of avian retrovirus RNA

Abstract

Cellular mRNAs from both animal (Banerjee, 1980) and plant (Nichols and Welder, 1981) cells and some viral mRNAs (Banerjee, 1980) contain, in addition to methylated nucleosides at their 5’ termini (cap structures of type m⁷G (5’) pppX^m), methylated bases at internal positions. Most of these internal methylations occur at the N-6 position of adenosine residues (m⁶A). There is considerable sequence specificity for these methylations and this specificity has been conserved throughout the animal and plant kingdoms (Nichols and Welder, 1981; Schibler et al., 1977, Wei and Moss, 1977; Dimock and Stoltzfus, 1977; Canaani et al., 1979). The functional role of m⁶A methylations has not yet been elucidated. As one approach to this problem, we have previously studied the effects of cycloleucine, an in vivo inhibitor of Ado Met synthesis (Caboche, 1977), on the biosynthesis of avian retroviruses (Dimock and Stoltzfus, 1978; Dimock and Stoltzfus, 1979). These results led us to conclude that the m⁶A and G^m methylations are not required for the synthesis of viral RNA and the transport of viral RNA to assembly sites. Because undermethylated chicken embryo fibroblast mRNA is associated with polyribosomes, we also concluded that such RNA is functional.