Gene Expression of Na+-K+-ATPase a1 and a3 Subunits in Gills of the Teleost Oreochromis mossambicus, Adapted to Different Environmental Salinities

The objective of the present study was to test the hypothesis that fish gills can express more than one isoform of the Na+-K+-ATPase a subunit responsible for ion regulation in seawater and freshwater environments. Using rapid amplification of complementary DNA ends (RACE), we cloned and sequenced full-length cDNAs encoding Na+-K+-ATPase a1 and a3 subunits of tilapia (Oreochromis mossambicus). Clone TG33 is 3390 bp in length and encodes a polypeptide of 1023 amino acids, while clone TH3 is 3581 bp in length and encodes a protein of 1010 amino acids. Clones TG33 and TH3 showed 91% and 88% identities at the amino acid level with previously described animal Na+-K+-ATPase a1 and a3 subunits, respectively. Northern blot and reverse transcriptase polymerase chain reaction analyses indicated that the a1 subunit is expressed predominantly in kidney and intestine, while the a3 subunit is expressed mainly in brain and heart. However, lower levels of expression of both genes were detected in other tissues such as gill, spleen, and testis. The amounts of both a1 and a3 subunit messenger RNA in gill tissue increased with the level of environmental salinity. This provides direct evidence of enhanced transcription of N+-K+-ATPase a1 and a3 subunit genes upon salinity challenge.