Kozak and IRES sequnces same thing?

Alan KAY kay at lyon151.inserm.fr
Wed Nov 5 06:03:39 EST 1997

In article <345FE918.851A796B at cyllene.uwa.edu.au>, mwatson at cyllene.uwa.edu.au 
>Hi All
>I am wondering what the differnce in function is between a Kozak
>sequence and an Internal Ribosome Entry Site (IRES).  Does anyone know?
>Do they do the same thing?  can one substitute for another?
>Thanks All
The answer to whether they are the same thing is no. The Kozak sequence 
defines a consensus sequence around an ATG codon which provides a good 
environement for the ATG codon to function as the translational initiation 
codon. Normally, a ribosome will attach to an mRNA 5' cap structure and then 
scan down the RNA until it encounters a suitable ATG codon, usually but not 
always in a Kozak environement, and then starts translation. When an IRES is 
used, the ribosome will attach directly to this internal structure (cap 
independant attachment), and then scan down until it finds a favourable ATG 
initiation codon. The IRES will therefore still need a good initiation codon 
(Kozak sequence usually) downstream, but a Kozak sequence does not 
necessarily need an IRES.
Alan KAY
"Virus des hepatites, retrovirus Humains et pathologies associees"
151 cours Albert Thomas, 69424 LYON CEDEX 03, FRANCE
e-mail : kay at lyon151.inserm.fr
tel : (33) 04 72 68 19 76
fax: (33) 04 72 68 19 71

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