Transcript mapping reveals different expression strategies for the bicistronic RNAs of the geminivirus wheat dwarf virus.

We have characterised the major transcripts of the Czech isolate of wheat dwarf virus (WDV-CJI) which show that WDV uses two different mechanisms for expressing overlapping open reading frames (ORFs). Mapping of the virion sense RNAs identified a single polyadenylated transcript of 1.1kb spanning the overlapping ORFs V1 and V2 which encode cell-cell spread functions and the coat protein respectively. This finding distinguishes WDV from other monocot-infecting geminiviruses studied so far which were shown to encode two 3' co-terminal transcripts capable of expressing either the V1 or V2 ORF. A survey of codon usage at the junction between the V1 and V2 ORF has led us to propose that translational frame shifting analogous to that in the yeast Ty element may occur. Analysis of polymerase chain reaction (PCR) amplified complementary sense cDNA clones has revealed the presence of mature spliced and unspliced RNAs which could encode products of an intron mediated C1:C2 ORF fusion or the C1 ORF product alone. Mapping of the 5' and 3' extremities of the major WDV encoded transcripts has allowed us to identify putative transcription regulatory sequences and the presence of multiple overlapping transcripts may suggest temporal regulation of transcription.