Observations concerning the discontinuous DNAs of cauliflower mosaic virus

The double-stranded circular DNA encapsidated within cauliflower mosaic virus (CaMV) particles contains three single-stranded discontinuities, two in one strand and one in the other, so that, upon denaturation, three linear single-stranded DNAs are produced. Here we show that a fourth much smaller single-stranded DNA, termed alpha1, is also present in denatured CaMV DNA preparations. The 5' extremity of alpha1 is identical to that of the alpha-strand, the strand of DNA possessing only one interruption, while its 3' extremity lies just two nucleotides downstream from a major transcription initiation site. We also show that the interrupted strand at each discontinuity sometimes has a single ribonucleotide in place of a deoxyribonucleotide at its 5' extremity. Oligoribonucleotide chains of eight and 10 residues in length have also been detected at the 5' end of one of the discontinuities. These structures are thought to be the vestiges of primers which have not been completely excised prior to encapsidation of the DNA. The possibility that synthesis of the alpha-strand occurs by reverse transcription of viral RNA using initiator tRNA(met) as primer is discussed.