HETEROGENEITIES IN VERTEBRATE TRANSFER RNASTRP AVIAN RETROVIRUSES PACKAGE ONLY AS A PRIMER THE TRANSFER RNATRP LACKING MODIFIED M2G IN POSITON-7

Bovine, rabbit and chicken tRNATrp species and tRNATrP packaged in avian myeloblastosis virus were separated and purified using twodimensionai gel electrophoresis and their primary structures were determined. Two major tRNATrp species (1 and 2) were identified in beef and rabbit, two minor ones (3 and 4) in beef and only one minor in rabbit. Their structures differ by 4 nucleotide substitutions located in the D, and T loops (positions 16, 47, 57 and 59). Species and 4 differ from one another by only one nucleotide at position 2. Differences between tRNATrp species were also observed in the extent of methylation of some nucleotides. Chicken tRNATrp presents only one species similar to the mammalian typel tRNATrp. In the case of the three studied animals this tRNA could be separated Into two subspecies, which differ by a post- transcriptional modification of nucleotide 7 in the acceptor stem : G or m2G. However only the non methylated species is used as the primer of DNA-RNA directed retrotranscription since it is only that form which was found in avian retroviruses. The methylation of G to m2G at position 7 could thus prevent the recognition of tRNATFP by retrovirai protein(s) responsible for the selective packaging of the primer tRNATrp. © 1990 Oxford University Press.