Abstract
The region of the SV-40 genome complementary to the 5'' end of the most abundant poly(A)-containing 19S and 16S mRNA was mapped by hybridization of double-labeled RNA ([3H]methyl group and [14C]uridine) to specific DNA fragments. Chemical identification of methylated residues indicated that a common leader sequence adjacent to the 5'' terminus of both 19S and 16S mRNA is transcribed from DNA sequences located between 0.67-0.76 map units. The estimated size of this leader RNA, which does not code for any known viral protein, is 170-200 nucleotides. Sequences complementary to the leader region and coding portion of 16S mRNA are apparently located in separate parts of the SV-40 genome.