MOLECULAR ANALYSIS OF THE ERWINIA-CHRYSANTHEMI REGION CONTAINING THE KDGA AND ZWF GENES

The pathways of pectin and galacturonate catabolism in Erwinia chrysan themi converge to form a common intermediate, 2-keto-3-deoxygluconate, which is phosphorylated to form 2-keto-3-deoxy-6-phosphogluconate (KD GP) and then cleaved by the aldolase encoded by the kdgA gene. We clon ed the kdgA gene of the E. chrysanthemi strain 3937 by complementing a n Escherichia coil kdgA mutation, using an RP4-derivative plasmid. Res triction mapping of the kdgA region and isolation of kdgA-lac fusions allowed the more precise localization of the kdgA gene and determinati on of its transcriptional direction. The nucleotide sequence of the kd gA region indicated that the kdgA reading frame is 639 bases long, cor responding to a protein of 213 amino acids with a molecular mass of 22 187 Da. Comparison of the deduced primary amino acid sequences of the E. chrysanthemi KDGP-aldolase to the E. coil, Zymomonas mobilis and Ps eudomonas putida enzymes showed that they are highly conserved. The E. chrysanthemi kdgA structural gene begins 153 bases downstream of an o pen reading frame that has a high homology with the zwf E. coil gene e ncoding glucose-6-phosphate dehydrogenase. The zwf gene is also linked to eda (kdgA) in E. coil and P. putida but genetic organization is di fferent. Regulation of zwf and kdgA expression in E. chrysanthemi was analysed using lacZ fusions. The expression of zwf is independent of t he growth rate, but is repressed in the presence of glucose. Induction of kdgA by pectin-degradation products is mediated in vivo by the neg ative regulatory gene kdgR, which also controls all the steps of pecti n degradation. However, the KdgR repressor is unable to bind in vitro to the 5'-untranslated end of the kdgA gene.