In this study, we characterized the duodenase-1 gene in fish (i.e.
tilapia) for the first time. We found that the duodenase-1 gene
encoded a serine protease with 252 amino acids residues in tilapia,
which is similar to that in mammals, such as 251 amino acids in
cattle [30]. But the amino acid sequence has a low similarity with
that of mammals (38.4e40%), suggesting that the evolution of this
gene is very rapid. There are differences in length of the signal
peptide in the gene in different animal species. The signal peptide is
composed of the first 17 residues in cattle, which shorter than that
(the first 22 residues) in tilapia. The activation di-peptide in tilapia
is SereAsp, and is preceded by a glycine residue. Generally, the
activation di-peptide in mammals is one of three main types
(GlyeLys, GlyeGlu, or GlueGlu). Therefore, activation peptides
differ in their structure between mammals and fish. Whether the
structure difference of the activation peptides influences the
functions of the gene requires further study. When tested for potential
positive selection of the gene in some cichlid species, we
found that the dN/dS values between different species were smaller
than 1, suggesting that this gene is under negative selection in
cichlid species, including tilapia. Duodenase is a serine protease. In
human, some genes encoding proteases were also under negative
selection [31]. Presumably, these protease genes have been subjected
to negative selection against autoactivation activity during
their evolution, because they have poor recognition of cleavage
sites [32]. The genomic structure of the duodenase-1 gene is very
similar to that of serine proteases with a common five exons and
four introns