Isolated yeast genes for amino acid biosynthesis provide ideal and powerful system for the investigation of such molecular genetic mechanisms as the control of gene expression, especially when the activity of gene products is not easily be monitored by classical biochemical approaches. THR4 yeast gene has been defined only through mendelian genetic analysis and assumed to encode for threonine synthase (THS), which should catalyse the last step of threonine biosynthesis pathway; interconversion of 0-phosphohomoserine and threonine. The enzyme THS has never been directly assayed or characterized. Several recombinant plasmids (pYL series) have been isolated from yeast genomic library (vector; 8.0kb YCp50) by complementation of thr4 mutation in a recipient yeast strain M21-59 (thr4 trp1 ura3). Digestion fragments by 29 restriction endoncleases were analyzed within the 7.5kb insert region of pYL3 (15.5kb), which appeared to contain the thr4 complementing DNA. Twenty-three recognition sites in the insert have been delineated for the following restriction enzymes: BamHI (1 site), ClaI (1), EcoRI (1), EcoRV (3), KpnI (1), NruI (1), NsiI (1), PstI (2), PvuII (2), SacI (2), SacII (1), SalI (2), XbaI (2) and XhoI (3).