Basso A L, Ricca E, Caruso C, Ferrara L, De Felice M
Istituto Adattamento Bovini e Bufali Ambiente Mezzogiorno, C.N.R., Naples, Italy.
Res Microbiol. 1993 Sep;144(7):539-45. doi: 10.1016/0923-2508(93)90003-k.
Acetohydroxy acid synthase (AHAS) and threonine deaminase (TD) activities were found in Streptococcus bovis and shown to be involved in the biosynthesis of the branched chain amino acids isoleucine, leucine and valine. Apparent lack of repression of AHAS synthesis by the end-products and reduced sensitivity of S. bovis growth to analogues of the branched chain amino acids suggested that secretion of isoleucine, leucine and valine in the growth medium may be a consequence of the regulatory features of AHAS. A glycyl-leucine-resistant mutant with reduced TD activity secreted a reduced amount of isoleucine and an increased amount of valine, which might be a result of the reduced rate of synthesis of the isoleucine precursor alpha-ketobutyrate and of a consequent preferential carbon flow through the valine branch of the pathway.
在牛链球菌中发现了乙酰羟酸合酶（AHAS）和苏氨酸脱氨酶（TD）的活性，且已表明它们参与支链氨基酸异亮氨酸、亮氨酸和缬氨酸的生物合成。终产物对AHAS合成明显缺乏抑制作用，以及牛链球菌生长对支链氨基酸类似物的敏感性降低，这表明在生长培养基中异亮氨酸、亮氨酸和缬氨酸的分泌可能是AHAS调节特性的结果。一个TD活性降低的甘氨酰 - 亮氨酸抗性突变体分泌的异亮氨酸量减少，缬氨酸量增加，这可能是异亮氨酸前体α - 酮丁酸合成速率降低以及随后碳流优先通过该途径缬氨酸分支的结果。
