Klebsiella pneumoniae genes for citrate lyase and citrate lyase ligase: localization, sequencing, and expression

Abstract

In the course of studies on anaerobic citrate metabolism in Klebsiella pneumoniae, the DNA region upstream of the gene for the sodium-dependent citrate carrier (citS) was investigated. Nucleotide sequence analysis revealed a cluster of five new genes that were oriented inversely to citS and probably form an operon. The genes were named citCDEFG. Based on known protein sequence data, the gene products derived from citD, citE and citF could be identified as the gamma-, beta-, and alpha-subunits of citrate lyase, respectively. This enzyme catalyses the cleavage of citrate to oxaloacetate and acetate. The gene product derived from citC (calculated M(r) 38,476) exhibited no obvious similarity to other proteins. In the presence of acetate and ATP, cell extracts from a citC-expressing Escherichia coli strain were able to reactivate purified citrate lyase from K. pneumoniae that had been inactivated by chemical deacetylation of the prosthetic group. This represents 5-phosphoribosyl-dephospho-acetyl-coenzyme A which is covalently bound to serine-14 of the acyl carrier protein (gamma-subunit). CitC was thus identified as acetate:SH-citrate lyase ligase. The function of the gene product derived from citG (M(r) 32,645) has not yet been identified. Expression of the citCDEFG gene cluster in E. coli led to the formation of citrate lyase which was active only in the presence of acetyl-coenzyme A, a compound known to substitute for the prosthetic group. These and other data strongly indicated that the enzyme synthesized in E. coli lacked its prosthetic group. Thus, additional genes besides citCDEFG appear to be required for the formation of holo-citrate lyase.