In vivo analysis of the TSWV cap-snatching mechanism: single base complementarity and primer length requirements.

Requirements for capped leader sequences for use during transcription initiation by tomato spotted wilt virus (TSWV) were tested using mutant alfalfa mosaic virus (AMV) RNAs as specific cap donors in transgenic Nicotiana tabacum plants expressing the AMV replicase proteins. Using a series of AMV RNA3 mutants modified in either the 5'-non-translated region or in the subgenomic RNA4 leader, sequence analysis revealed that cleaved leader lengths could vary between 13 and 18 nucleotides. Cleavage occurred preferentially at an A residue, suggesting a requirement for a single base complementarity with the TSWV RNA template, which could be confirmed by analyses of host mRNAs used in vivo as cap donors.