Genomic structure of nitric oxide synthase in the terrestrial slug is highly conserved

Nitric oxide (NO) is a key molecule in olfactory information processing across animal species. To gain insight into the genetic basis of NO generation, we investigated the genomic structure of nitric oxide synthase (NOS) in the terrestrial slug Limax because slugs use olfaction as their primary food detection system. The full length cDNA of limNOS encodes a protein consisting of 1632 amino acids that has a PSD-95/discs-large/ZO-1 (PDZ) domain in its N-terminus and 6 other cofactor-binding domains. The limNOS gene consists of 33 exons and spans at least 107 kb of the genome. Almost all the exon-intron boundaries are conserved in Limax and human nNOS and the organization of the Limax genome is more similar to that of humans than to Drosophila, indicating that there was an accelerated evolution of the Drosophila genome during evolution. These results imply that there was a highly conservative selective pressure imposed on NOS gene structure during the evolution of mollusks and vertebrates.