MAPPING AND EXPRESSION OF SOUTHERN BEAN MOSAIC-VIRUS GENOMIC AND SUBGENOMIC RNAS

The coat protein of the cowpea strain of southern bean mosaic sobemovi rus (SBMV-C) is translated from a subgenomic RNA (sgRNA) that is synth esized in the virus-infected cell. Like the SBMV-C genomic RNA, the sg RNA has a viral protein (VPg) covalently bound to its 5' end. The mech anism(s) by which ribosomes initiate translation on the SBMV-C RNAs is not known. To begin to characterize the translation of the sgRNA it w as first necessary to precisely map its 5' end. Primer extension was u sed to identify SBMV-C nucleotide (nt) 3241 as the transcription start site. As a control, the 5' end of the genomic RNA was also mapped. Su rprisingly, the 5' terminal nt of this RNA was identified as SBMV-C nt 2. The primary structure of the 5' ends of these two RNAs is therefor e expected to be VPg-ACAAAA. Precise mapping of the 5' end of the sgRN A of the bean strain of SBMV (SBMV-B) demonstrated that it has these s ame elements. Translation of coal protein from the SBMV-C sgRNA and p2 1 from the SBMV-C genomic RNA was compared using a cell-free system. T he results of these experiments were consistent with translation of th ese proteins by a 5' end-dependent scanning mechanism rather than by i nternal ribosome binding. (C) 1997 Academic Press.