Biology Department, Concordia University, 1455 de Maisonneuve Ave., Montreal, Quebec H3G 1M8, Canada.
J Bacteriol. 2010 Oct;192(20):5515-25. doi: 10.1128/JB.00748-10. Epub 2010 Aug 20.
Escherichia coli K-12 provided with glucose and a mixture of amino acids depletes L-serine more quickly than any other amino acid even in the presence of ammonium sulfate. A mutant without three 4Fe4S L-serine deaminases (SdaA, SdaB, and TdcG) of E. coli K-12 is unable to do this. The high level of L-serine that accumulates when such a mutant is exposed to amino acid mixtures starves the cells for C(1) units and interferes with cell wall synthesis. We suggest that at high concentrations, L-serine decreases synthesis of UDP-N-acetylmuramate-L-alanine by the murC-encoded ligase, weakening the cell wall and producing misshapen cells and lysis. The inhibition by high L-serine is overcome in several ways: by a large concentration of L-alanine, by overproducing MurC together with a low concentration of L-alanine, and by overproducing FtsW, thus promoting septal assembly and also by overexpression of the glycine cleavage operon. S-Adenosylmethionine reduces lysis and allows an extensive increase in biomass without improving cell division. This suggests that E. coli has a metabolic trigger for cell division. Without that reaction, if no other inhibition occurs, other metabolic functions can continue and cells can elongate and replicate their DNA, reaching at least 180 times their usual length, but cannot divide.
大肠杆菌 K-12 在提供葡萄糖和混合氨基酸的情况下，即使存在硫酸铵，也比任何其他氨基酸更快地耗尽 L-丝氨酸。大肠杆菌 K-12 中没有三种 4Fe4S L-丝氨酸脱氨酶（SdaA、SdaB 和 TdcG）的突变体无法做到这一点。当这样的突变体暴露在氨基酸混合物中时，会积累大量的 L-丝氨酸，使细胞饥饿 C(1) 单位并干扰细胞壁合成。我们认为，在高浓度下，L-丝氨酸会降低由 murC 编码的连接酶合成 UDP-N-乙酰胞壁酸-L-丙氨酸，从而削弱细胞壁并产生畸形细胞和裂解。通过几种方式克服高浓度 L-丝氨酸的抑制作用：通过大量的 L-丙氨酸、与低浓度 L-丙氨酸一起过量产生 MurC 以及过量产生 FtsW，从而促进隔膜组装，还可以通过糖裂解操纵子的过度表达。S-腺苷甲硫氨酸可减少裂解并允许大量增加生物量，而不会改善细胞分裂。这表明大肠杆菌有一种用于细胞分裂的代谢触发因素。如果没有该反应，在没有其他抑制发生的情况下，其他代谢功能可以继续进行，细胞可以伸长并复制其 DNA，达到至少 180 倍的通常长度，但不能分裂。
