Characteristics of a satellite-like virus of tobacco ringspot virus

Abstract

A heterogeneous population of viral particles (SL-TRSV), some of which sediments almost as fast (122 S) as the bottom component (126 S) of tobacco ringspot virus (TRSV), was discovered as an unexpected part of a mixed infection with TRSV. Although SL-TRSV appears to be dependent upon TRSV for its own replication, SL-TRSV commonly becomes the major portion of the virus population. SL-TRSV is not as homogeneous as either middle or bottom component of TRSV, but rather sediments with a range of rates between middle and bottom component of TRSV (91–126 S); yet the nucleic acid extracted from SL-TRSV is homogeneous and sediments at only 7.3 S in contrast to the nucleic acids extracted from TRSV, which sediment at 24 S and 32 S. The 7.3 S nucleic acid of SL-TRSV appears to be single-stranded and RNase sensitive. Thus, according to Gierer's formula, a molecular weight of only 86,000 daltons is indicated for the nucleic acid of SL-TRSV. Both SL-TRSV and TRSV are indistinguishable in size and shape and in serological tests. SL-TRSV was not detected in plants inoculated either with the 7.3 S nucleic acid alone or with TRSV (strain ST) alone, but was detected in plants inoculated with a mixture of the 7.3 S nucleic acid and TRSV. Lesions in blackeye cowpeas resulting from inoculation with the mixture were distinct from those induced by TRSV alone. The 7.3 S nucleic acid alone induced no visible lesions at 5 μg/ml; but the 7.3 S nucleic acid at 0.004 μg/ml, in combination with TRSV, induced some tiny lesions typical of SL-TRSV. These results suggest that SL-TRSV cannot replicate in the absence of TRSV; and that each capsid of SL-TRSV, which may be identical to a capsid of TRSV, can contain from 14 to 26 strands of nucleic acid of uniform length. The nucleic acid strands within the capsid, or extracted from the capsid, are biologically active when used in inoculum together with TRSV.