Splicing in a plant pararetrovirus

Abstract

Analysis of expression in rice cells of plasmids in which an ATG-less chloramphenicol acetyl transferase ORF was placed in frame with the coding sequence of the pararetrovirus rice tungro bacilliform virus (RTBV) ORF IV, and which contained much of the upstream full-length transcript sequence of RTBV, gave evidence to suggest that a splicing event occurred. Reverse transcription/PCR of RNA from transfected protoplasts and from RTBV-infected plants yielded a product which confirmed that the 5' end of ORF IV was spliced in frame to a short ORF (sORF) in the RTBV leader sequence removing an intron of about 6.3 kb. Most of the translation is initiated at the ATG codon in the sORF with only about 10% at the ORF IV ATG codon. The efficiency of splicing appears to be inversely related to the length of the intron. The finding of splicing in a pararetrovirus blurs the differences between them and retroviruses but is in accord with the hypothesis that retroelements acquire genes and sequences which adapt them to specific niches.