Trace amount of satellite RNA associated with tobacco ringspot virus: increase stimulated by nonaccumulating satellite RNA mutants.

The small satellite RNA of tobacco ringspot virus (sTRSV RNA) is dependent on tobacco ringspot virus (TRSV) for replication and encapsidation. sTRSV RNA has appeared during serial passage of certain TRSV strains in some hosts. Co-inoculation of bean with TRSV and either of two related, nonaccumulating mutants of sTRSV RNA induced the appearance of sTRSV RNA in a single passage (van Tol et al., 1991, Virology 180, 23-30). The sTRSV RNA obtained after serial passage and after co-inoculation have the same nucleotide sequence, designated the endogenous sequence. The endogenous sTRSV RNA nucleotide sequence differs from that of each of the nonaccumulating sTRSV RNA at three positions. In order to detect possible trace amounts of endogenous satellite RNA in virion RNA preparations, RNA from two TRSV isolates was subjected to reverse transcription and polymerase chain reaction of the transcript (RT-PCR), using primers with sTRSV RNA terminal sequences. The yield of RT-PCR product suggests that the virion RNA preparations contained approximately 0.1 fg of sTRSV RNA per microgram of virion RNA. The nucleotide sequence of the RT-PCR product corresponded to that of the endogenous sTRSV RNA. The endogenous sTRSV RNA of TRSV inocula appears to be latent, being maintained in very small amounts during serial passage of TRSV in some hosts but capable of dramatic increase during serial passage in other hosts or when TRSV was co-inoculated with either of two specific sTRSV RNA mutants. Ten other nonaccumulating sTRSV RNA mutants did not induce a detected increase in sTRSV RNA.