A NOVEL, TESTIS-SPECIFIC MESSENGER-RNA TRANSCRIPT ENCODING AN NH2-TERMINAL TRUNCATED NITRIC-OXIDE SYNTHASE

mRNA diversity represents a major theme of neuronal nitric-oxide synth ase (nNOS) gene expression in somatic cells/tissues. Given that gonads often express unique and biologically informative variants of complex genes, we determined whether unique variants of nNOS are expressed in the testis. Analysis of cDNA clones isolated from human testis identi fied a novel, testis-specific nNOS (TnNOS) mRNA transcript. A predicte d 3294-base pair open reading frame encodes an NH2-terminal truncated protein of 1098 amino acids. Measurement of calcium-activated L-[C-14] citrulline formation and nitric oxide release in CHO-K1 cells stably t ransfected with the TnNOS cDNA indicates that this protein is a calciu m-dependent nitric-oxide synthase with catalytic activity comparable t o that of full-length nNOS. TnNOS transcripts exhibit novel 5' mRNA se quences encoded by two unique exons spliced to exon 4 of the full-leng th nNOS. Characterization of the genomic structure indicates that exon ic regions used by the novel TnNOS are expressed from intron 3 of the NOS1 gene. Although lacking canonical TATA and CAAT boxes, the 5'-flan king region of the TnNOS exon 1 contains multiple putative cis regulat ory elements including those implicated in testis-specific gene expres sion. The downstream promoter of the human nNOS gene, which directs te stis-specific expression of a novel NH2-terminal truncated nitric-oxid e synthase, represents the first reported example in the NOS gene fami ly of transcriptional diversity producing a variant NOS protein.