Ultraviolet mapping of RNA transcripts encoded by the cauliflower mosaic virus genome

The transcription of the cauliflower mosaic virus (CaMV) genome was analyzed by ultraviolet (uv) mapping procedures to determine promoter sites and polarity of synthesis of "late" viral RNA transcripts. The late transcripts include two major, discrete polyadenylated RNAs, a large full-genome-length transcript and a 19 S RNA, and nonpolyadenylated RNA fragments smaller than the full-genome-length transcript. All late transcripts are derived from one strand of the CaMV genome, the alpha strand. When protoplasts were irradiated with increasing uv doses, the ability of the CaMV genome to support the synthesis of viral RNA was lost in a polar fashion. The EcoR1 fragments of the CaMV genome in order of decreasing uv sensitivity were a1, a2, c, then b. The uv sensitivity data coupled with other observations are most consistent with a model in which CaMV transcription is promoted from two sites-one near EcoR1-d and the other counterclockwise to the single-strand interruption in the alpha strand in the BglII subfragment c of EcoR1-b. The data do not fit a model in which transcription is promoted from one site, just clockwise from the alpha strand interruption site.