Over‐production, Purification and Properties of the Uridine‐diphosphate‐N ‐Acetylmuramate: l‐alanine Ligase from Escherichia coli

The UDP-N -acetylmuramate:l-alanine ligase of Escherichia coli was over-produced in strains harbouring recombinant plasmids bearing the murC gene under the control of the lac or trc promoter. Plasmid pAM1005, in which the promoter and ribosome-binding site region of murC were removed and in which the gene was directly under the control of promoter trc, led to a 2000-fold amplification of the l-alanine-adding activity after induction by isopropyl-thio-β-D-galactopyranoside. The murC gene product was visualized as a 50-kDa protein accounting for approximately 50% of the cell protein. A two-step purification led to 1 g of a homogeneous protein from an 18–1 culture. The N-terminal sequence of the purified protein correlated with the nucleotide sequence of the murC gene. The presence of 2-mercaptoethanol and glycerol was essential for the stability of the enzyme. The Km values for UDP-TV-acetylmuramic acid, l-alanine and ATP/Mg2+ were estimated at 100, 20 and 450 μM, respectively. Under the optimal in vitro conditions a turnover number of 928 min−1 was calculated and a copy number/cell of 600 could be roughly estimated. The specificity of the enzyme for its substrates was investigated with various analogues. The enzyme also catalysed the reverse reaction.