bicaudal encodes the Drosophila beta NAC homolog, a component of the ribosomal translational machinery

bicaudal was the first Drosophila mutation identified as producing mirror-i mage pattern duplications along the anteroposterior axis of the embryo. How ever the mutation has been little studied due to its low penetrance and sup pressibility, We undertook cloning of the bicaudal locus together with stud ies of the mutation's effects on key elements of the posterior embryonic pa tterning pathway. Our mapping studies place the bicaudal mutation within a similar to 2 kb region, 3' to the protein coding sequence of the Drosophila homolog of beta NAG, a subunit of Nascent polypeptide Associated Complex ( NAC), Genomic DNA encoding beta NAC completely rescues the bicaudal phenoty pe, The lethal phenotype of Enhancer of Bicaudal, E(Bic), a mutation hypoth esized to affect the bicaudal locus, is also completely rescued by the beta NAC locus. We further demonstrate that the E(Bic) mutation is caused by a P element insertion into the transcribed region of the beta NAC gene. NAC i s among the first ribosome-associated entities to bind the nascent polypept ide after peptide bond formation. In contrast to other bicaudal-embryo-prod ucing mutations, bicaudal leads to ectopic translation of mRNA for the post erior determinant nanos, without affecting the localization of mRNA for its upstream regulator, oskar, in the embryo. These findings suggest that repr ession of nanos mRNA translation occurs on the ribosome and involves a role for beta NAG.