COMPARISON OF VIRAL NUCLEIC-ACID INTERMEDIATES AT EARLY AND LATE STAGES OF CAULIFLOWER MOSAIC-VIRUS INFECTION SUGGESTS A FEEDBACK REGULATORY MECHANISM

An important phase of the multiplication cycle of the pararetrovirus cauliflower mosaic virus (CaMV) is transcription of the viral minichromosome in the nucleus. Leaves of infected turnip plants at the vein clearing stage were found to contain a relatively low level of minichromosome DNA, and abundant viral transcripts and characteristic reverse transcription products. In contrast, at the much later stage of severe leaf chlorosis, an elevated level of minichromosome DNA but less RNA, especially the 35S RNA reverse transcription template, was observed. Changes in the composition of virus nucleic acid intermediates were also seen in roots and stems early, compared with late, in infection. A possible feedback mechanism controlling the level of viral minichromosome DNA and its importance in regulation of the CaMV multiplication cycle are discussed in the light of these observations.