Localization of the 5' terminus of late SV40 mRNA.

A cap-containing oligonucleotide has been isolated from a T1 ribonuclease hydrolysate of total Simian Virus 40-specific late RNA and its structure has been determined as 7mG(5')ppp(5') mAmpU(m)pUp(Up,Cp)ApGp. This oligonucleotide constitutes the major 5' terminus of both late mRNA species, 16S and 19S. Assuming that viral mRNA "caps" are derived from a 5'-terminal triphosphate or diphosphate, these results mean that late transcription is (mainly) initiated at nucleotide L 308 (numbering system of Simian Virus 40 DNA, cf. Fiers et al. (1978) Nature, 273, 113-120). The cap is followed by a contiguous leader sequence which is at least 194 nucleotides long.