Characterization of a gene encoding a basic protein of the spermatid nucleus, TNP2, and its close linkage to the protamine genes in the bull.

During elongation and condensation of the spermatid nucleus, histones are replaced by spermatid-specific transition proteins (TNP). TNP1 is well characterized at the cDNA and at the genomic level and was found to be highly conserved during mammalian evolution (similarity between 83 to 98%). We here describe for the first time the nucleotide sequence and organization of the gene for TNP2. The gene was isolated from a bull cosmid library and was found to contain a single intron of 910 bp. The coding sequence consists of 390 bp and has a similarity of about 70% to that of the TNP2 cDNAs of mouse and rat. At the basis of amino-acid sequences, the bull TNP2 is 14 and 15 amino acids longer than that of mouse and rat, respectively, and the similarity is only 45% between bull and mouse and 42% between bull and rat. However, the evolutionary divergence has not occurred at the cost of basic amino acids which are of functional importance in DNA-protein interaction in the condensing spermatid nucleus. The TNP2 gene is closely linked to the protamine genes in the bull genome.