A ribosomal binding site (RBS) is a sequence on mRNA that is bound by the ribosome

Ribosome

A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....

 when initiating protein translation.

It can be either the 5' cap

5' cap

The 5' cap is a specially altered nucleotide on the 5' end of precursor messenger RNA and some other primary RNA transcripts as found in eukaryotes. The process of 5' capping is vital to creating mature messenger RNA, which is then able to undergo translation...

 of a messenger RNA in eukaryotes, a region 6-7 nucleotides upstream of the start codon

Start codon

The start codon is generally defined as the point, sequence, at which a ribosome begins to translate a sequence of RNA into amino acids.When an RNA transcript is "read" from the 5' carbon to the 3' carbon by the ribosome the start codon is the first codon on which the tRNA bound to Met,...

 AUG in prokaryotes (called the Shine-Dalgarno sequence

Shine-Dalgarno sequence

The Shine-Dalgarno sequence , proposed by Australian scientists John Shine and Lynn Dalgarno , is a ribosomal binding site in the mRNA, generally located 8 basepairs upstream of the start codon AUG. The Shine-Dalgarno sequence exists only in prokaryotes. The six-base consensus sequence is AGGAGG;...

), or an internal ribosome entry site

Internal ribosome entry site

An internal ribosome entry site, abbreviated IRES, is a nucleotide sequence that allows for translation initiation in the middle of a messenger RNA sequence as part of the greater process of protein synthesis...

 (IRES) in viruses. The sequence is complementary to the 3' end of the rRNA. The ribosome

Ribosome

A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....

 searches for this site and binds to it through base-pairing of nucleotides. Then, the ribosome begins the translation process and recruits initiation factors. After finding the ribosome binding site in eukaryotes, the ribosome recognizes the Kozak consensus sequence

Kozak consensus sequence

The Kozak consensus sequence, Kozak consensus or Kozak sequence, is a sequence which occurs on eukaryotic mRNA and has the consensus gccRccAUGG, where R is a purine three bases upstream of the start codon , which is followed by another 'G'. The Kozak consensus sequence plays a major role in the...

and begins translation at the +1 AUG codon.