Programmed Ribosomal frameshifting 
is an alternate mechanism of translation to merge proteins encoded by two overlapping open reading frames. The frameshift occurs at low frequency and consists of ribosomes slipping by one base in either the 5'(-1) or 3'(+1) directions during translation. Some viruses contains both a +1 and a -1 ribosomal frameshifts
-1 Ribosomal frameshift
All cis-acting frameshift signals encoded in mRNAs are minimally composed of two functional elements: a heptanucleotide "slippery sequence" conforming to the general form X XXY YYZ, followed by a RNA structural element, usually a H-type RNA pseudoknot, positioned an optimal number of nucleotides (5 to 9) downstream. Mechanistically, the ribosome is stalled on the slippery sequence by the pseudoknot structure in 3'. In about 10% cases, the ribosome will backtrack one nucleotide, creating one or more mismatchs between tRNAs and mRNA. Translation resolves on the backtracked ribosome in the -1 frame.
Source:Michael Bekaert, thesis (in french) etude du decalage de phase de lecture dans le genome de Saccharomyces cerevisiae
+1 ribosomal frameshift
This frameshift is mechanistically different from the -1. The ribosome stalls on a slippery sequence, making a long pause at a rare codon for which few tRNAs are available. This long pause can be resolved by a movement forward of one nucleotide, creating one or more mismatchs between tRNAs and mRNA. Translation resolves on the advanced ribosome in the +1 frame.
An insect nidovirus emerging from a primary tropical rainforest
Florian Zirkel, Andreas Kurth, Phenix-Lan Quan, Thomas Briese, Heinz Ellerbrok, Georg Pauli, Fabian H. Leendertz, W. Ian Lipkin, John Ziebuhr, Christian Drosten, Sandra Junglen
MBio 2011; 2: e00077-00011
NS1? of flaviviruses in the Japanese encephalitis virus serogroup is a product of ribosomal frameshifting and plays a role in viral neuroinvasiveness
Ezequiel Balmori Melian, Edward Hinzman, Tomoko Nagasaki, Andrew E. Firth, Norma M. Wills, Amanda S. Nouwens, Bradley J. Blitvich, Jason Leung, Anneke Funk, John F. Atkins, Roy Hall, Alexander A. Khromykh
J. Virol. February 2010; 84: 1641?1647
Discovery of frameshifting in Alphavirus 6K resolves a 20-year enigma
Andrew E. Firth, Betty Yw Chung, Marina N. Fleeton, John F. Atkins
Virol. J. 2008; 5: 108
NS1? of flaviviruses in the Japanese encephalitis virus serogroup is a product of ribosomal frameshifting and plays a role in viral neuroinvasiveness
Ezequiel Balmori Melian, Edward Hinzman, Tomoko Nagasaki, Andrew E. Firth, Norma M. Wills, Amanda S. Nouwens, Bradley J. Blitvich, Jason Leung, Anneke Funk, John F. Atkins, Roy Hall, Alexander A. Khromykh
J. Virol. February 2010; 84: 1641?1647
