Fragments of the African cassava mosaic virus (ACMV) genome, cloned upstream of the β-glucuronidase (GUS) reporter gene in an expression cassette, were analysed for their ability to direct complementary-sense gene expression in tobacco protoplasts by measuring GUS activity. Five arbitrary domains (A-E) have been designated that contribute to the expression of AC1 (replication-associated protein) and AC4. Consistent with earlier reports, gene expression was negatively regulated (80% reduction in activity) by its own protein product, and suppression was mimicked by truncated versions of AC1 comprising the N-terminal 57 amino acids. AC1 also suppressed gene expression to a similar extent. Nucleotide sequences responsible for suppression were mapped to domain A, a 92 bp fragment located immediately upstream of the AC1 initiation codon encompassing the consensus TATA box and transcription start point. Complementary-sense gene expression also decreased by 30–40% in the presence of AV1 (coat protein) although other DNA A-encoded proteins (AV2, AC2, AC3 and AC4) had no effect. The results are discussed in the light of recent advances concerning the initiation of viral DNA replication and the control of gene expression.


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