EVIDENCE OF RNA RECOMBINATION IN THE GENOME 3'-TERMINAL REGION OF PAV-LIKE ISOLATES OF BARLEY YELLOW DWARF VIRUS (BYDV-PAV)

The genome 3'-terminal region of the PAV-serotype of barley yellow dwa rf virus (BYDV-PAV) covers 2 subgenomic RNAs (sgRNA2 and sgRNA3). The sgRNA2 is responsible for the expression of the ORF6 (ORF: open readin g frame). The sgRNA3 corresponds to the 334-terminal nucleotides and d oes not carry coding sequences. In a previous study, we compared the n ucleotide sequences of the genome 3'-terminal region for 10 BYDV-PAV i solates differing in their geographical origins and biological propert ies. In the present investigation we show that the sequence homology g rouping obtained for the 5' half of this region is different from that of the 3' half. Therefore, some isolates that are grouped in differen t clusters according to sequence homologies observed for the 5' half m ay be grouped in the same cluster according to the 3' half. These diff erences in sequence homology grouping suggest either different pressur es of selection or RNA recombination. The hypothesis of RNA recombinat ion between the 5' half of ancestors of some BYDV-PAV isolates and the 3' half of ancestors of other isolates, leading to isolates differing in their grouping according to both halves, is more favourable. This essentially relies on the fact that the 3' half of the genome 3'-termi nal region covers the sgRNA3. The sgRNA3 may have some promoters or st ructures on its 5' terminus. Being easily recognised by the RNA polyme rase, these structures may facilitate RNA recombination by strand swit ching during replication in mixed infection.