STRUCTURAL REQUIREMENTS FOR EFFICIENT TRANSLATIONAL FRAMESHIFTING IN THE SYNTHESIS OF THE PUTATIVE VIRAL RNA-DEPENDENT RNA-POLYMERASE OF POTATO LEAFROLL VIRUS

The putative RNA-dependent RNA polymerase of potato leafroll luteoviru s (PLRV) is expressed by - 1 ribosomal frameshifting in the region whe re the open reading frames (ORF) of proteins 2a and 2b overlap. The si gnal responsible for efficient frameshift is composed of the slippery site UUUAAAU followed by a sequence that has the potential to adopt tw o alternative folding patterns, either a structure involving a pseudok not, or a simple stem-loop structure. To investigate the structure req uirements for efficient frameshifting, mutants in the stem-loop or in the potential pseudoknot regions of a Polish isolate of PLRV (PLRV-P) have been analyzed. Mutations that are located in the second stem (S2) of the potential pseudoknot structure, but are located in unpaired re gions of the alternative stem-loop structure, reduce frameshift effici ency. Deletion of the 3' end sequence of the alternative stem-loop str ucture does not reduce frameshift efficiency. Our results confirm that - 1 frameshift in the overlap region depends on the slippery site and on the downstream positioned sequence, and propose that in PLRV-P a p seudoknot is required for efficient frameshifting. These results are i n agreement with those recently published for the closely related beet western yellows luteovirus (BWYV).