A novel class of large and infectious defective RNAs of Citrus tristeza virus.

Citrus tristeza virus (CTV)-infected plants contain one or more populations of defective RNAs (dRNAs), mostly with a size range of ca. 2.0 to 5.0 kb. Several CTV dRNAs have been characterized and found to consist mainly of the two termini of the genomic RNA, with extensive internal deletions. The present paper describes a new class of large ( approximately 12.0 kb) dRNAs from three different CTV isolates with two unusual features. First is their composition with intact replicase genes. These dRNAs contained a large 5' portion of the genomic RNA terminus, which apparently corresponded to the recently described 5' large single-stranded subgenomic RNA (sgRNA) of ORF1a+1b (Che et al., 2001). The 3' portion of the large dRNAs varied among the 10 different cDNA clones examined in this work. In 2 dRNAs this portion consisted of truncated ORF10 (p20), and in 5 dRNAs it contained truncated ORF11 (p23). Two dRNA molecules were found with a 3' portion that started in the exact 5' position of the intergenic region between the p20 and p23 ORFs. In one dRNA, this portion coincided with the full-length sgRNA corresponding to ORF10. The second unusual feature was their ability to be readily transmitted mechanically to citrus plants by stem slashing and also to Nicotiana benthamiana protoplasts. The possibility that these dRNAs may be encapsidated and be capable of self-replication is discussed.