RICE RAGGED STUNT ORYZAVIRUS GENOME SEGMENT S4 COULD ENCODE AN RNA-DEPENDENT RNA-POLYMERASE AND A 2ND PROTEIN OF UNKNOWN FUNCTION

The complete nucleotide sequence of genome segment S4 of rice ragged s tunt oryzavirus (RRSV, Thai-isolate) was determined. The 3823 bp seque nce contains two large open reading frames (ORFs). ORF1, spanning nucl eotides 12 to 3776, is capable of encoding a protein of M-r 141,380 (P 4a). The P4a amino acid sequence predicted from the nucleotide sequenc e contains sequence motifs conserved in RNA-dependent RNA polymerases (RDRPs). When compared for evolutionary relationships with RDRPs of ot her reoviruses using the amino acid sequences around the conserved GDD motif, P4a was shown to be more related to Nilaparvata lugens reoviru s and reovirus serotype 3 than to rice dwarf phytoreovirus, bovine rot avirus or bluetongue virus. The ORF2, spanning nucleotides 491 to 1468 , is out of frame with ORF1 and is capable of encoding a protein of 36 , 920 (P4b). Coupled in vitro transcription-translation from cloned OR F2 in wheat germ extract confirmed the existence of ORF2 but in vivo p roduction and possible function of P4b is yet to be determined.