The cotton-top tamarin carries an extended semenogelin I gene but no semenogelin II gene
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Previous studies have shown that the predominant proteins secreted by the seminal vesicles are transglutaminase substrates which have undergone major structural alterations during evolution. In man, they are known as semenogelin I and II; recently it was shown that, similar to man, several new world and old world monkeys carry two semenogelin genes as well, the exception being the cotton-top tamarin (Saguinus oedipus) with a single gene. This gene has now been cloned and identified as a semenogelin I gene, because of a higher number of conserved nucleotides in the human semenogelin I gene (89%) than in the human and the rhesus monkey semenogelin II genes (82%). Furthermore, the difference in sequence similarity indicates that the semenogelin II gene was deleted from the genome of a progenitor to the cotton-top tamarin after the duplication that yielded the two semenogelin genes seen in man. Like several other genes expressing seminal-vesicle-secreted transglutaminase substrates, the cotton-top tamarin semenogelin I gene consists of three exons of 97, 1816 and 146 bp. It codes for a signal peptide of 23 amino acid residues and the secreted protein of 592 amino acid residues. The molecular mass of 66 kDa is 32% larger than that of the human counterpart and, contrary to human semenogelin I, the cotton-top tamarin protein has the potential to be highly glycosylated as there are 14 sites with the consensus sequence for N-linked glycosylation. Approximately half of the primary structure consists of five nearly identical tandem repeats of 58 amino acid residues, that probably evolved relatively late.