Amplification of human argininosuccinate synthetase pseudogenes

The human genome contains multiple pseudogenes for an argininosuccinate synthetase (AS) gene. To elucidate the molecular mechanisms of generation and dispersion, complete nucleotide sequences of four different AS pseudogenes, psi AS-Y, psi AS-A1, psi AS-A2 and psi AS-A3, have been determined. A comparison of these sequences with those of three reported AS pseudogenes, psi AS-1, psi AS-3 and psi AS-7 revealed that two pairs, psi AS-Y/psi AS-7 and psi AS-A3/psi AS-1, are highly homologous but not identical, thereby suggesting that one of the pairs is generated by a duplication of the other member of the pairs. The psi AS-Y, which is probably located on chromosome Y, and the partially sequenced psi AS-7 are both interrupted by an Alu element at exactly the same site in their 3'-end regions. These two Alu elements are located in an opposite orientation relative to the direction of transcription of the pseudogene, and their possible role on pseudogene dispersion was examined. The psi AS-A1 is also accompanied by an Alu element at its 3' end. In this case, the orientation of the Alu element is the same as that of the pseudogene. The psi AS-A1 and the Alu element are flanked with direct repeats, as if they had been inserted into a chromosomal site, as a single unit.