Identification of aecA mutations in Bacillus subtilis as nucleotide substitutions in the untranslated leader region of the aspartokinase II operon

Abstract

Recent genetic mapping of the aspartokinase II (lysC) operon of Bacillus subtilis [M. Petricek. L. Rutberg & L. Hederstedt (1989) FEMS Microbiology Letters 61,85-88; N. Y. Chen. J. J. Zhang & H. Paulus (1989) Journal of General Microbiology 135, 2931-2940] has shown its chromosomal location to be close to the aecA locus, the mutation of which leads to highly increased levels of aspartokinase II. In order to examine the relationship between lysC and aecA, we have cloned the control regions of the lysC operon from several independent aecA mutants and determined their nucleotide sequences. The nucleotide sequences of the aecA mutants differed from the wild-type sequence by the substitution of one or two nucleotides at two widely separated sites in the transcribed leader region of the lysC operon. To confirm that the observed nucleotide changes are indeed responsible for the AecA phenotype and not simply the reflection of sequence polymorphisms in different B. subtilis strains, we introduced the same nucleotide substitutions as those observed in the aecA strains into the leader region of the wild-type lysC operon by oligonucleotide-directed mutagenesis. The expression of the mutagenized genes was analysed after transcriptional or translational fusion to lacZ in a single-copy integration vector. The levels of β-galactosidase were greatly elevated by the nucleotide substitutions, with similar increases observed in transcriptional and translational fusions. The high level of expression of β-galactosidase in the lysC-lac'Z strains with nucleotide substitutions corresponding to the aecA mutations was resistant to repression by L-lysine but was completely abolished by the inactivation of the lysC promoter. These results suggest that the sites identified by nucleotide sequencing of the aecA mutants are important regulatory sites in the lysC operon, which exert their effect at the level of transcription involving the normal lysC promoter.