Cucumber mosaic virus RNA 5 Partial characterization and evidence for no large sequence homologies with genomic RNAs

Abstract

Cucumber mosaic virus (CMV) contains four major RNA species (RNAs 1,2,3 and 4) the largest three of which are necessary for infectivity [ 1,2] . RNA 4 serves as a messenger for synthesis of coat protein in a wheat embryo cell-free system [3] . The nucleotide sequence of RNA 4 must also be present in RNA 3 since it is known that RNA 3 carries the information for coat protein synthesis in vivo [4,5] . In addition to the genome RNAs and RNA 4, a fifth RNA component (RNA 5) of low molecular weight (100 000) has been detected in CMV preparations [ 1,6] . Interest in RNA 5 was enhanced by the finding that, even though the small RNA is apparently dependent upon the CMV genome RNAs for replication [7] , the relative proportions of RNA 5 and RNA l-4 is tied to the nature of the plant host of viral propagation and, furthermore, the presence of RNA 5 can dramatically alter the response of certain hosts to CMV infection [8,9]. The above findings make it unlikely that RNA 5 is a host RNA normally present in the plant which is entrapped during virus assembly, but they do not in themselves permit us to distinguish between two further possibilities: (1) RNA 5 originates from the CMV RNA but is nonessential for infectivity, i.e., a deletion fragment or partial transcription product of one of the genome RNAs, or (2) RNA 5 is a ‘satellite’ RNA, that is, a parasite RNA with little or no sequence homology with the helper genome. As a prelude to a more detailed sequence analysis, we have determined some of the properties of RNA 5. In particular, the 5’ and 3’ extremities of the RNA