GENOME ORGANIZATION AND GENE-EXPRESSION OF SAGUARO CACTUS CARMOVIRUS

The complete sequence of the single-stranded, (+)-sense RNA genome of saguaro cactus carmovirus (SCV) has been determined. The 3879 nucleoti de genome contains five open reading frames (ORFs). The 5'-proximal OR F encodes a 26 kDa protein (p26) and terminates with an amber codon wh ich is readthrough into an in-frame p57 ORF to generate an 86 kDa fusi on protein (p86). Two small, centrally located ORFs encode a 6 kDa pro tein (p6) and a 9 kDa protein (p9), respectively. The 3'-proximal ORF encodes a 37 kDa (p37) capsid protein (CP). Analysis of the nucleotide and predicted amino acid sequences supports the classification of SCV in the genus Carmovirus in the family Tombusviridae. All predicted SC V proteins are expressed in an in vitro translation system. SCV p26 an d the readthrough fusion protein p86 are synthesized from the genomic RNA while p6, p9 and p37 CP ORFs at the 3' half of the genome are expr essed from two subgenomic (sg) RNAs. The 5' termini of both sg RNAs ha ve been mapped. The large 1614 nucleotide sg RNA contains the p6 and p 9 ORFs as the first and the second ORFs respectively from its 5' end. It directs the synthesis of abundant p6 but a small amount of p9. Whil e a synthetic transcript with the p9 ORF at the 5' end is a more effic ient messenger for p9, no corresponding sg RNA has been identified in vivo. The smaller 1396 nucleotide sg RNA contains only the p37 ORF and directs the synthesis of SCV CP.