• malonate decarboxylase;
  • Klebsiella pneumoniae;
  • cosmid library;
  • DNA sequence;
  • expression in Escherichia coli.

Malonate decarboxylase of Klebsiella pneumoniae consists of four different subunits and catalyzes the conversion of malonate plus H+ to acetate and CO2. The catalysis proceeds via acetyl and malonyl thioester residues with the phosphribosyl-dephospho-CoA prosthetic group of the acyl carrier protein (ACP) subunit. From a cosmid library of K. pneumoniae, a gene cluster of 9 kb has been isolated and sequenced that included the structural genes for the malonate decarboxylase. The cluster consisted of the eight consecutive genes mdcABCDEFGH and the divergently oriented mdcR gene. The intergenic regions were short (usually < 17 bp, 136 bp between mdcE and mdcF) and ribosome binding sites were found 4-lObp before each gene. According to N-terminal protein sequencing, the mdcA, C, D and E genes encoded subunits α, δ, β and γ of malonate decarboxylase. Data bank searches for related proteins with known function revealed that MdcA represents the ACP-transferase and that MdcD and E together probably function as malonyl-S-ACP decarboxylase. MdcC is the (apo) ACP subunit. MdcB and MdcG could be involved in the synthesis and attachment of the prosthetic group. MdcH is similar to various malonyl-CoA:ACP-SH transacylases and therefore probably involved in the initial activation of the enzyme by malonylation. MdcF is a membrane protein that could function as a malonate carrier. The mdcR gene encodes a protein of the LysR regulator family. Malonate decarboxylase was functionally expressed in Escherichia coli from plasmids harbouring the entire gene cluster including mdcR. As partial deletion of the mdcR gene impaired growth of the transformants on malonate, MdcR is probably a transcriptional regulator of the mdc genes.