Abstract
Branched-chain aminotransferase (BCT) catalyzes the reversible transamination of the branched-chain alpha-keto acids to the branched-chain L-amino acids. Since branched-chain L-amino acids (L-isoleucine, L-leucine, and L-valine) are essential for cell growth, cells which lack BCT were unable to proliferate in media containing alpha-keto acids in place of the corresponding L-amino acids. CHW-1102, a Chinese hamster cell line, lacks BCT and does not grow in alpha-keto acid media. Somatic cell hybrids were made by the fusion of CHW-1102 (HPRT-) with several human cell lines and isolated on HAT medium. Growth assays of hybrid clones on alpha-keto acid selection media independent of the HAT selection medium indicated two cell hybrid phenotypes: either (1) the hybrid clone, like the parental CHW-1102, could not utilize alpha-keto acid media, or (2) the hybrid could proliferate on all three alpha-keto acid media. The ability of hybrid cells to proliferate on alpha-keto acid media correlated with the presence of either of two human genes which independently complemented the Chinese hamster deficiency. Two human genes. BCT1 assigned to chromosome 12 and BCT2 assigned to chromosome 19, were demonstrated to code for the expression of two molecular forms of BCT.
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