Construction of full-length cDNA clones of lettuce mosaic virus (LMV) and the effects of intron-insertion on their viability in Escherichia coli and on their infectivity to plants

A full length cDNA copy of the genomic RNA of lettuce mosaic virus (LMV) wa s constructed under the control of an enhanced CaMV 35S promoter and of the NOS terminator. This construct was found infectious when inoculated to let tuce plants. The intron II of the bean nitrite reductase gene was engineere d into the LMV FL cDNA in order to relieve possible deleterious effects of viral sequences to Escherichia coli cells and to evaluate the effects of th e presence of the intron on the FL cDNA infectivity. The intron-less FL cDN A was found to be as stable as its intron-containing counterpart in E. coli . Sequence analysis of progeny RNA derived from plants inoculated with the intron-containing FL cDNA demonstrated that the inserted intron was perfect ly spliced out. The symptoms induced in lettuce by either the intron-less o r the intron-containing constructs were identical to those caused by the wi ld-type virus. However a slight delay in the establishment of infection in lettuce and a more obvious lag in Nicotiana benthamiana were observed with the intron-containing FL cDNA.