The Drosophila nitric-oxide synthase gene (dNOS) encodes a family of proteins that can modulate NOS activity by acting as dominant negative regulators

Nitric oxide (NO) is involved in organ development, synaptogenesis, and res ponse to hypoxia in Drosophila. We cloned and analyzed the only gene in the fly genome that encodes Drosophila nitric-oxide synthase (dNOS). It consis ts of 19 exons and is dispersed over 34 kilobases of genomic DNA. Alternati ve transcription start sites and alternative splice sites are used to gener ate a remarkable variety of mRNAs from the dNOS gene. We identified eight n ew transcripts that are widely expressed throughout Drosophila development and encode a family of DNOS-related proteins. Alternative splicing affects both the 5'-untranslated region and the coding region of the dNOS primary t ranscript. Most of the splicing alterations in the coding region of the gen e lead to premature termination of the open reading frame. As a result, non e of the alternative transcripts encode an enzymatically active protein. Ho wever, some of these shorter DNOS protein products can effectively inhibit enzymatic activity of the full-length DNOS1 protein when co-expressed in ma mmalian cells, thus acting as dominant negative regulators of NO synthesis. Using immunoprecipitation, we demonstrate that these short DNOS protein is oforms can form heterodimers with DNOS1, pointing to a physical basis for t he dominant negative effect. Our results suggest a novel regulatory functio n for the family of proteins encoded by the Drosophila NOS gene.