SPLICING FEATURES IN MAIZE STREAK VIRUS VIRION-SENSE AND COMPLEMENTARY-SENSE GENE-EXPRESSION

The single-stranded DNA geminiviruses produce transcripts from both st rands (virion-and complementary-sense) of a nuclear double-stranded DN A molecule. In maize streak virus (MSV)-infected maize plants, approxi mately 80% of the complementary-sense transcripts produce the C1 prote in, whilst the remaining 20% are spliced to remove a 92 nt intron and produce a C1:C2 fusion protein (Rep). Disruption of the complementary- sense 3' splice site abolished virus replication. The majority of the virion-sense transcripts initiated one nucleotide upstream of the V1 ( movement protein) gene and a minority a further 141 nucleotides upstre am. A 76 nt intron, with features typical of plant introns, was identi fied within the V1 gene, upstream of the coat protein gene. Spliced an d unspliced forms of each virion-sense transcript were produced, but t hey differed in splicing efficiency. Approximately 50% of the major tr anscript and less than 10% of the minor transcript were processed. Mut agenesis of the consensus 5' splice site in the V1 gene resulted in th e use of alternative cryptic splice sites, confirming the importance o f splicing for MSV infection. Spliced virion-sense transcripts were al so identified in tissue infected with the closely-related Digitaria st reak virus (DSV) but not with another subgroup I geminivirus, wheat dw arf virus. Collectively, the multiple transcript initiation sites and different splicing efficiencies suggest that splicing is an important feature in the regulation of both early and late gene expression in MS V and DSV.