Additional file 1: Coding sequences of the synthetic genes tbage and tneub. Coding sequences of the synthetic genes tbage and tneub. The sequences are provided in FASTA format. The XbaI site is underlined, and the NsiI site is double-underlined. The start codon ATG and the stop codon TAA are presented in bold letters.  mRNA pseudoknot structures can act as ribosomal roadblocks Several viruses utilize programmed ribosomal frameshifting mediated by mRNA pseudoknots in combination with a slippery sequence to produce a well defined stochiometric ratio of the upstream encoded to the downstream-encoded protein. A correlation between the mechanical strength of mRNA pseudoknots and frameshifting efficiency has previously been found; however, the physical mechanism behind frameshifting still remains to be fully understood. In this study, we utilized synthetic sequences predicted to form mRNA pseudoknot-like structures. Surprisingly, the structures predicted to be strongest lead only to limited frameshifting. Two-dimensional gel electrophoresis of pulse labelled proteins revealed that a significant fraction of the ribosomes were frameshifted but unable to pass the pseudoknot-like structures. Hence, pseudoknots can act as ribosomal roadblocks, prohibiting a significant fraction of the frameshifted ribosomes from reaching the downstream stop codon. The stronger the pseudoknot the larger the frameshifting efficiency and the larger its roadblocking effect. The maximal amount of full-length frameshifted product is produced from a structure where those two effects are balanced. Taking ribosomal roadblocking into account is a prerequisite for formulating correct frameshifting hypotheses. The reading frame of the vast majority of mRNAs is determined by the start codon after which the downstream cistron is translated in the same frame. Maintenance of the reading frame occurs without further signals to the ribosome. However, examples of genes containing information for programmed frameshifts can be found in most organisms, or in some of their IS sequences, transposable elements, retroelement-derived sequences or viruses. The sequence-information needed for programmed ribosomal frameshift varies and both +1 and À1 frameshifts can be induced (1) (2) (3) .