EXPRESSION OF THE 2ND LYSINE DECARBOXYLASE GENE OF ESCHERICHIA-COLI

Certain amino acids are substrates for two decarboxylase enzymes in Es cherichia coil, one inducible by anaerobic growth at low pH and the ot her constitutive. In the case of lysine, an inducible decarboxylase (C adA) has been extensively characterized, but evidence for the existenc e of a second lysine decarboxylase is fragmentary and uncertain. This paper confirms that a second lysine decarboxylase is encoded by a locu s (Idc) previously suggested to be a lysine decarboxylase gene on the basis of sequence comparisons. Overexpression of the cloned gene provi ded sufficient quantities of enzyme in cell-free extracts for prelimin ary examination of the properties of the Idc gene product, Ldc. The en zyme is active over a broad range of pH with an optimum at 7.6, much h igher than that of CadA, about 5.5. The temperature optimum for both e nzymes is similar, at about 52 degrees C, but Ldc is more readily inac tivated by heat than CadA. Expression of Idc from its own promoter was very weak for cells growing in a variety of media, although a low lev el of lysine decarboxylase was present in cells that carried the Idc r egion on an oligo-copy plasmid when these were grown in minimal-glucos e medium. Northern analysis of RNA extracted from such cells revealed a transcript whose length corresponded to that of the Idc gene, sugges ting that Ide is normally transcribed from a promoter immediately upst ream. However, most of the Idc mRNA was shorter, indicating degradatio n or premature termination. The Ide upstream sequence promoted transcr iption of a lacZ gene to which it was fused. Introduction of the upstr eam sequence as an insert in a multicopy vector increased transcriptio n of the resident lacZ fusion. The low level of expression in single c opy, the emergence of expression when the gene is present at moderate copy number, and the derepression by the upstream sequence in trans im ply that this second lysine decarboxylase gene may not be constitutive but subject to specific repression by a factor which remains to be id entified.