INTERNAL TRANSLATIONAL INITIATION IN THE MESSENGER-RNA FROM THE NEUROSPORA-CRASSA ALBINO-3 GENE

The ''ribosome scanning model'' for translational initiation predicts that eukaryotic mRNAs should, as a rule, be monocistronic. However, ca ses have recently been described of eukaryotic mRNAs producing more th an one protein through alternative translational initiation at several different AUG codons. The present work reports the occurrence of mult iple translational start sites on the mRNA of the Neurospora crassa ge ne albino-3 (al-3), encoding the carotenoid biosynthetic enzyme gerany lgeranyl-pyrophosphate synthase. This was revealed by the molecular an alysis of an al-3 mutant carrying a deletion within the coding sequenc e, which was expected to prevent the synthesis of a functional geranyl geranyl-pyrophosphate synthase because of ribosome frameshifting and p remature translational termination. However, the mutants could maintai n appreciable geranylgeranyl-pyrophosphate synthase activity through a mechanism operating at the translational level, whereby a fraction of ribosomes initiated protein synthesis from either of two internal in- frame AUG codons located downstream of the deletion, thus producing a shortened but still active version of the geranylgeranyl-pyrophosphate synthase. The results presented indicate that the internal AUG codons are recognized mainly or solely by direct ribosome binding rather tha n by ''leaky scanning'' from the 5' end of the mRNA.