sentence: Recombinant pHIS2 and empty AD ( control ) vectors were transformed into yeast strain Y187 ( Clontech Laboratories , Mountain View , CA , United States ) , which was grown on SD screening medium ( SD / - Leu-Trp-His ) containing different concentrations of 3-amino-1 ,2,4 - triazole ( 3-AT ) , an appropriate concentration of which suppresses background histidine leakiness in pHIS2 vectors .