NEURONAL TYPE INFORMATION ENCODED IN THE BASIC-HELIX-LOOP-HELIX DOMAIN OF PRONEURAL GENES

The proneural genes encode basic-helix-loop-helix (bHLH) proteins and promote the formation of distinct types of sensory organs. In Drosophi la, two sets of proneural genes, atonal (ate) and members of the achae te-scute complex (ASC), are required for the formation of chordotonal (ch) organs and external sensory (es) organs, respectively. We assayed the production of sensory organs in transgenic flies expressing chime ric genes of ato and scute (sc), a member of ASC, and found that the i nformation that specifies ch organs resides in the bHLH domain of ate; chimeras containing the b domain of ato and the HLH domain of sc also induced ch organ formation, but to a lesser extent than those contain ing the bHLH domain of ate. The b domains of ato and sc differ in seve n residues, Mutations of these seven residues in the b domain of ato s uggest that most or perhaps all of these residues are required for ind uction of ch organs. None of these seven residues is predicted to cont act DNA directly by computer simulation using the structure of the myo genic factor MyoD as a model, implying that interaction of ato with ot her cofactors is likely to be involved in neuronal type specification.