Complete inhibition of tobamovirus multiplication by simultaneous mutations in two homologous host genes

The TOM1 gene of Arabidopsis thaliana encodes a putative multipass transmembrane protein which is necessary for the efficient multiplication of tobamoviruses. We have previously shown that mutations severely destructive to the TOM1 gene reduce tobamovirus multiplication to low levels but do not impair it completely. In this report, we subjected one of the tom1 mutants (tom1-1) to another round of mutagenesis and isolated a new mutant which did not permit a detectable level of tobamovirus multiplication. In addition to tom1-1, this mutant carried a mutation referred to as tom3-1. Positional cloning showed that TOM3 was one of two TOM1-like genes in Arabidopsis. Based on the similarity between the amino acid sequences of TOMI and TOM3, together with the results of a Sos recruitment assay suggesting that both TOMI and TOM3 bind tobamovirus-encoded replication proteins, we propose that TOMI and TOM3 play parallel and essential roles in the replication of tobamoviruses.