[Sequence aanlysis of intergenic region of rice stripe virus RNA4: evidence for mixed infection and genomic variation].

The intergenic region (IR) of the RNA4 of 22 isolates of Rice stripe virus (RSV) in China was cloned and sequenced. The IR sequences were compared with one another and with that from Japan. Sequence comparisons showed that these isolates could be divided into three different types, with the IR length of 634 bp, 654 bp and 732 bp, respectively. It is interesting to note three different types all occurred in Yunnan RSV natural population, whereas other province only existed 654 bp type length isolates. Mixed infections with different types of IR length coexisting in some isolates in Yunnan was observed. IR sequences were not more conserved (83% - 100%) among the populations of RSV from China than with those of RSV isolates from Japan (83% - 94%). There were two important structure characteristics in IRs sequences. Firstly, there was a-19 nt insertion in 654 bp type isolates and a-103 nt in 732 bp type isolates in comparison to 634 bp type isolates. This inserted sequences were rather highly conserved. Blast analysis indicates the 16 nt (AGAAACATGAGAGTA) in 19 nt insertation was very similar in sequence to wheat cDNA library; and the 20 nt (AGAATTGCCTTGGTGTTAT) in 103 nt insertion was identical to a stretch sequences of barley cDNA library. Recombination hot-spot sequences existed in RNA4 IR. Secondly, IRs sequence was rich in U and A residues where two distant hairpin structures could be formed with computer-assisted folding analysis. One was highly conserved and stable, but the other was rather unstable because of bases variation. It is believed that this stabilised hairpin structure, rather than a sequence motif, might serve as a transcription terminator during the synthesis of mRNAs from the ambisense segments. Negative selection constraints imposed by secondary structure might have maintained the conserved sequences. In this paper, the relationship between the lowest free energy of the unstable hairpin structures and the different pathogenesis among some isolates was also discussed in this paper.