Martin R R, Marshall V D, Sokatch J R, Unger L
J Bacteriol. 1973 Jul;115(1):198-204. doi: 10.1128/jb.115.1.198-204.1973.
Two types of Pseudomonas putida PpG2 mutants which were unable to degrade branched-chain amino acids were isolated after mutagenesis and selection for ability to grow on succinate, but not valine, as a sole source of carbon. These isolates were characterized by growth on the three branched-chain amino acids (valine, isoleucine, and leucine), on the corresponding branched-chain keto acids (2-ketoisovalerate, 2-keto-3-methylvalerate, and 2-ketoisocaproate), and on other selected intermediates as carbon sources, and by their enzymatic composition. One group of mutants lost 2-ketoisovalerate-inducible branched-chain keto acid dehydrogenase that was active on all three keto acids. There was also a concomitant loss of ability to grow on all three branched-chain amino acids as well as on all three corresponding keto acids, but there was retention of ability to use subsequent intermediates in the catabolism of branched-chain amino acids. Another type of mutant showed a marked reduction in branched-chain amino acid transaminase activity and grew poorly at the expense of all three amino acids, but it utilized subsequent intermediates as carbon sources. Both the transaminase and branched-chain keto acid dehydrogenase mutants retained the ability to degrade camphor. These findings are consistent with the view that branched-chain amino acid transaminase and branched-chain keto acid dehydrogenase are common enzymes in the catabolism of valine, isoleucine, and leucine.
通过诱变并筛选出能够以琥珀酸而非缬氨酸作为唯一碳源生长的能力后，分离出了两种不能降解支链氨基酸的恶臭假单胞菌PpG2突变体。这些分离菌株的特征在于，它们能够以三种支链氨基酸（缬氨酸、异亮氨酸和亮氨酸）、相应的支链酮酸（2-酮异戊酸、2-酮-3-甲基戊酸和2-酮异己酸）以及其他选定的中间产物作为碳源生长，同时还对它们的酶组成进行了研究。一组突变体失去了对所有三种酮酸都有活性的2-酮异戊酸诱导型支链酮酸脱氢酶。同时，它们失去了以所有三种支链氨基酸以及所有三种相应酮酸为碳源生长的能力，但保留了在支链氨基酸分解代谢过程中利用后续中间产物的能力。另一类突变体的支链氨基酸转氨酶活性显著降低，以所有三种氨基酸为碳源生长时生长较差，但它能够利用后续中间产物作为碳源。转氨酶突变体和支链酮酸脱氢酶突变体都保留了降解樟脑的能力。这些发现与以下观点一致，即支链氨基酸转氨酶和支链酮酸脱氢酶是缬氨酸、异亮氨酸和亮氨酸分解代谢中的常见酶。
