Cloning,Purification, and Characterization of Diaminobutyrate Acetyltransferase from the Halotolerant Methanotroph <Emphasis Type="Italic">Methylomicrobium alcaliphilum</Emphasis> 20Z

L-2,4-Diaminobutyrate (DAB) acetyltransferase (DABAcT) catalyzes one of the key reactions of biosynthesis of the bacterial osmoprotectant ectoine--acetylation of L-2,4-DAB yielding Ngamma-acetyl-2,4-DAB. Gene ectA encoding DABAcT was cloned from DNA of the halotolerant methanotroph Methylomicrobium alcaliphilum 20Z and expressed in Escherichia coli with an additional six His residues at the C-terminus. Homogeneous enzyme preparation with specific activity 200 U/mg was obtained by affinity metal-chelating chromatography. DABAcT was found to be a homodimer with molecular mass 40 kD. The enzyme is most active at pH 9.5 and 20 degrees C, and its activity increased threefold in the presence of 0.1-0.2 M NaCl or 0.2 M KCl. The Km values of recombinant DABAcT measured at the optimal pH and temperature in the presence of 0.2 M KCl were 460 and 36.6 microM for L-2,4-DAB and acetyl-CoA, respectively. The enzyme is specific for L-2,4-DAB and acetyl-CoA and is also active against propionyl-CoA (20%). Zn2+ and Cd2+ at 1 mM concentration completely inhibit the recombinant enzyme; 10 mM ATP inhibits 26% of the enzyme activity, whereas EDTA, o-phenanthroline, ADP, NAD(P), and NAD(P)H do not significantly effect the enzyme activity. The possible participation of DABAcT in regulation of ectoine biosynthesis in M. alcaliphilum 20Z is discussed.