REGULATION OF AFRICAN-CASSAVA-MOSAIC-VIRUS COMPLEMENTARY-SENSE GENE-EXPRESSION BY N-TERMINAL SEQUENCES OF THE REPLICATION-ASSOCIATED PROTEIN AC1

Fragments of the African cassava mosaic virus (ACMV) genome, cloned up stream of the beta-glucuronidase (GUS) reporter gene in an expression cassette, were analysed for their ability to direct complementary-sens e gene expression in tobacco protoplasts by measuring GUS activity. Fi ve arbitrary domains (A-E) have been designated that contribute to the expression of ACI (replication-associated protein) and AC4. Consisten t with earlier reports, AC1 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 5 7 amino acids. AC1 also suppressed AC4 gene expression to a similar ex tent. Nucleotide sequences responsible for suppression were mapped to domain A, a 92 bp fragment located immediately upstream of the AC1 ini tiation codon encompassing the consensus TATA box and transcription st art point. Complementary-sense gene expression also decreased by 30-40 % in the presence of AV1 (coat protein) although other DNA A-encoded p roteins (AV2, AC2, AC3 and AC4) had no effect. The results are discuss ed in the light of recent advances concerning the initiation of viral DNA replication and the control of gene expression.