Kozak and IRES sequnces same thing?

John Richard Seavitt jrseavit at artsci.wustl.edu
Wed Nov 5 12:18:29 EST 1997

On Wed, 5 Nov 1997, Dr Mark Watson wrote:

> 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?

Kozak sequences are a mRNA feature that have a distinct sequence
(ACCACCATGG or so) that appear to be involved in the initiation of
translation in a cap-dependent fashion. IRES are largely a viral
adaptation that allows them to bypass cap-dependent translation (which
relevant virii promptly disable to remove host cell demands on metabolic
demands and host cell anti-viral responses).  An IRES is a structurally
complex bit of work which mimics the structure of the cap complex, but not
the primary sequence.

They 'do the same thing' in that they are both involved in the initiation
of translation.  They do not make identical contributions to such
inititation, obviously.  A kozak site can not subsititute for an IRES, and
although a IRES could functionally substitute for a kozak site, the
regulation of translation would be quite distinct.

IRES are frequently used to generate bicistronic genes that code
independently for two gene products from a single mRNA species (for
example, your gene of interest and selection marker).  The IRES is placed
between yfg and the selection marker to allow cap-independent translation
of that marker under the overall control of whatever promoter is being

John Seavitt

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