Cryptic Initiation Sites in Eukaryotic mRNAs

Cryptic Initiation Sites in Eukaryotic mRNAs

Cryptic Initiation Sites in Eukaryotic mRNAs - Prokaryotic mRNAs are polycistronic and can initiate protein synthesis at internal positions. On the other hand eukaryote mRNAs are monocistronic. They have been described as containing only one initiation site for polypeptide chain by Jacobson and Baltimore (1968), who suggested that in animal cells a single m RNA produces a single polypeptide chain (one mRNA, one polypeptide hypothesis).

(In some cases a polyprotein is produced, which later undergoes cleavage to form several proteins).

That eukaryotic mRNA can contain more than one initiation site was first indicated from the work on the brome mosaic virus (BMV), a plant virus. Subsequently, internal initiation sites have been found in the mRNAs of many eukaryotic plant and animal viruses, ego TMV, TY MV, Semliki Forest Virus (SMV) and avian and murine RNA tumour viruses.

These sites are, however, inactive or cryptic in protein synthesis. Although eukaryote mRNAs can have internal polypeptide initiation sites, only the site nearer to the 5' end is active. Thus the general rule of Jacobson and Baltimore that one mRNA directs the synthesis of only one polypeptide chain is still valid.

However, in view of the discovery of internal initiation sites the rule is better expressed as follows: "one mRNA has only one active initiation site and the active site is always that which is nearest to the 5' end"

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