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Nitric oxide (NO) is a potent intercellular signal in mammals that mediates key aspects of blood pressure, hormone release, nerve transmission and the immune response of higher organisms. Proteins homologous to full-length mammalian nitric oxide synthases (NOSs) are found in lower multicellular organisms. Recently, genome sequencing has shown that some bacteria contain genes coding for truncated NOS proteins; this is consistent with reports of NOS-like activities in bacterial extracts. Biological functions for bacterial NOSs are unknown, but have been presumed to be analogous to their role in mammals. Here we describe a gene in the plant pathogen Streptomyces turgidiscabies that encodes a NOS homologue, and we reveal its role in nitrating a dipeptide phytotoxin required for plant pathogenicity. High similarity between bacterial NOSs indicates a general function in biosynthetic nitration; thus, bacterial NOSs constitute a new class of enzymes. Here we show that the primary function of Streptomyces NOS is radically different from that of mammalian NOS. Surprisingly, mammalian NO signalling and bacterial biosynthetic nitration share an evolutionary origin.

In most moth species the female sex pheromone evaporates from the surface of a gland that is everted and exposed to the air1–3. Considerable attention has been directed towards the pheromone biology of arctiid moths because (1) the female pheromone gland is atypical, consisting of a branched tube or pair of tubes that is invaginated into the haemocoel and opens dorsally between the eight and ninth abdominal segments4–5, and (2) pheromone release behaviour ('calling') is also atypical, involving rhythmic protrusion and retraction of the abdominal tip4. This pulsing action, which alternately exposes and occludes the orifice of the pheromone gland, is thought to result in the expulsion of discrete puffs of pheromone in the vapour phase4. We now report that calling females of the arctiid species, Pyrrharctia isabella, emit a visible stream of liquid droplets. This emission consists solely of the sex pheromone, with no incidental material or inert carrier material present in the aerosol. The quantities of pheromone released in this way are the largest known for a female moth. Also this is the first report of an airborne sex-attractant released as a liquid.