RT-PCR detection of chimeric MStV mRNA 4 sequences. Total RNA from individual healthy plants (lane 1), MStV-infected plants (lane 2), MStV- + BSMV-infected plants (lane 3), BSMV-infected plants (lane 4), plus mixed RNAs from MStV- and BSMV-infected plants (lane 5) was converted to cDNA using primer MStV50 (see Table 1). The cDNA was amplified using primers 5′MStV-CON and 3′NCP (A) to detect MStV RNA 4; the following primer sets: 5′BSMV1 and 3′NCP (B), 5′BSMV2 and 3′NCP (C), or 5′BSMV3 and 3′NCP (D) were used in attempts to detect BSMV/MStV chimeric RNAs. One-tenth of the PCR reaction was electrophoretically separated in 3.5% Metaphore agarose followed by staining with ethidium bromide. Sizes of fragments were estimated by using the BRL 1-kb DNA ladder (not shown).