Figure 5.

Hypothetical way to increase the copy number of the tandem repeat during the reverse transcriptase step of TPV genome replication. Based on the position of the RNA polymerase II promoter/leader after ORF4 in the closely related CsVMV (25), it is assumed that the tandem repeat containing two copies of the 63-bp monomer (two blocks) in the putative TPV leader will be present in the terminal repeats of the slightly greater than genome-length TPV RNA (dotted line). Using tRNA (cloverleaf) as a primer at nucleotide 1 of TPV DNA, reverse transcriptase (RT) synthesizes minus strand DNA (solid line) and degrades the RNA template until it reaches the terminal redundancy, where the DNA hybridizes to the complementary RNA sequence before RT switches strands. Because of the presence of the repeat in this region, there could be misalignment in the hybrid (oppositely pointing arrows), leading to the addition of one copy to produce three copies of the monomer. No misalignment maintains two copies. A_n signifies the poly(A) tail on the RNA.