The putative RNA-dependent RNA polymerase of potato leafroll luteovirus (PLRV) is expressed by -1 rribosomal frameshifting In the region where the open reading frames (ORF) of proteins 2a and 2b overlap. The signal responsible for efficient frameshlft is composed of the slippery site UUUAAAU followed by a sequence that has the potential to adopt two alternative folding patterns, either a structure Involving a pseudoknot, or a simple stem-loop structure. To investigate the structure requirements 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 regions of the alternative stem-loop structure, reduce frameshlft efficiency. Deletion of the 3’ end sequence of the alternative stem-loop structure does not reduce frameshlft efficiency. Our results confirm that —1 frameshlft in the overlap region depends on the slippery site and on the downstream positioned sequence, and propose that in PLRV-P a pseudoknot Is required for efficient frameshifting. These results are in agreement with those recently published for the closely related beet western yellows luteovirus (BWYV).