Identification of Bombyx Atonal and Functional Comparison with the Drosophila Atonal Proneural Factor in the Developing Fly Eye

The proneural genes are fundamental regulators of neuronal development in all metazoans. A critical role of the fly proneural factor Atonal (Ato(Dm)) is to induce photoreceptor neuron formation in Drosophila, whereas its murine homolog, Atonal7(Mm) (aka Ath5) is essential for the development of the ganglion cells of the vertebrate eye. Here, we identify the Bombyx mori ato homolog (ato(Bm) ). In a pattern strikingly reminiscent of ato(Dm), the ato(Bm) mRNA is expressed as a stripe in the silkworm eye disc. Its DNA-binding and protein-protein interaction domain is highly homologous to the Ato(Dm) bHLH. Targeted expression of Ato(Bm) in the endogenous ato(Dm) pattern rescues the eyeless phenotype of the fly ato(1) mutant and its ectopic expression induces similar gain-of-function phenotypes as Ato(Dm). Rescue experiments with chimeric proteins show that the non-bHLH portion of Ato(Bm) (N-region) can effectively substitute for the corresponding region of the fly transcription factor, even though no apparent conservation can be found at the amino acid level. On the contrary, the highly similar bHLH domain of Ato(Bm) cannot similarly substitute for the corresponding region of Ato(Dm). Thus, the bHLH(Bm) domain requires the Ato(Bm) N-region to function effectively, whereas the bHLH(Dm) domain can operate well with either N-region. These findings suggest a role for the non-bHLH portion of Ato proteins in modulating the function of the bHLH domain in eye neurogenesis and implicate specific aa residues of the bHLH in this process.