Aspartokinase genes lysC alpha and lysC beta overlap and are adjacent to the aspartate beta-semialdehyde dehydrogenase gene asd in Corynebacterium glutamicum

Abstract

A 2.1 kb DNA fragment of the recombinant plasmid pCS2, isolated from an aminoethyl cysteine (AEC)-resistant and lysine-producing Corynebacterium glutamicum mutant strain, and which confers AEC resistance and lysine production on the wild-type G. glutamicum ATCC 13032 was analysed. DNA sequence analysis of this fragment revealed three large open reading frames (ORFs). The incomplete ORF1 does not contain the 5' end of the coding region. ORF2, which uses the same reading frame as ORF1, is identical to the 3' end of ORF1 and encodes a putative protein of 172 amino acids (aa) and of Mr 18,584. ORF3 encodes a putative protein of 344 aa and of Mr 36,275. The amino acid sequences deduced from ORF1 and ORF2 display strong homologies to those of the alpha- and beta-subunits of the Bacillus subtilis aspartokinase II. It is therefore proposed that the incomplete ORF1, termed lysC alpha, encodes part of the alpha-subunit of the C. glutamicum aspartokinase whereas the complete ORF2, termed lysC beta, encodes the beta-subunit of the same enzyme. ORF2 is responsible for AEC resistance and lysine production due to a feedback-resistant aspartokinase. The amino acid sequence deduced from ORF3, termed asd, is highly homologous to that of the Streptococcus mutans aspartate beta-semialdehyde dehydrogenase (ASD). Plasmids carrying the C. glutamicum asd gene complemented Escherichia coli asd mutants. Increase in ASD activity by a factor of 30-60 was measured for C. glutamicum cells harbouring high copy-number plasmids with the C. glutamicum asd gene.