Transcription from the CaMV 19 S promoter and autocatalysis of translation from CaMV RNA.

The 19 S promoter of cauliflower mosaic virus was analyzed in host protoplasts. In spite of its weakness, it contains a fully functional core promoter. It can be strongly activated by 35 S enhancer elements, even when these elements are located downstream of it, comparable to the situation in the viral genome. The 19 S promoter also contains an element that can strongly enhance expression from a heterologous core promoter. A plasmid expressing the same CAT ORF from two overlapping transcription units, a dicistronic one under control of the 35 S promoter and a monocistronic one under control of the 19 S promoter, was constructed. While in the absence of the virus ORF VI product (pVI, "transactivator") only low levels of CAT activity deriving from the 19 S promoter were observed, in the presence of this protein high levels of CAT activity derived from the 35 S unit were observed in addition. This suggests autocatalytic activation of pVI expression during virus infection.