ANALYSIS OF THE LEUB GENE FROM CORYNEBACTERIUM-GLUTAMICUM

The leuB gene of Corynebacterium glutamicum was found to be present on a 2.2-kb BamHI-SacI chromosomal fragment which complemented the leuB mutation of Escherichia coli. The activity of 3-isopropylmalate dehydr ogenase (EC 1.1.1.85), encoded by the leuB gene, was significantly inc reased in C. glutamicum cells harbouring a plasmid containing the 2.2- kb fragment. The nucleotide sequence of the C. glutamicum leuB coding region (an open reading frame, ORF, of 1020 bp encoding a polypeptide of 340 amino acids with M-r of 36 144) was determined. The deduced ami no acid sequence of the product of this ORF is highly homologous to th ose of 3-isopropylmalate dehydrogenases from three species of mycobact eria. The transcriptional start site of the leuB gene was localized 35 bp upstream of its translational start; a functional terminator was d etected in the 3' flanking region. Northern hybridization analysis sho wed that the C. glutamicum leuB gene is transcribed as a single monoci stronic RNA (approximately 1.2 kb in size). Activity of the leuB promo ter was significantly reduced when leucine was present in the growth m edium. This suggests the negative regulation of the leuB expression on the transcriptional level in C. glutamicum cells.