A moderate amplification of the mecB gene encoding cystathionine-γ-lyase stimulates cephalosporin biosynthesis in Acremonium chrysogenum

L-cysteine is a precursor of the penicillin, cephalosporin and cephamycin families of beta -lactam antibiotics. Cystathionine-gamma -lyase (encoded by the mecB gene), an enzyme that splits cystathionine releasing: cysteine, is required for high-level cephalosporin production in methionine-supplemented medium. By amplification of the mecB gene in Acremonium chrysogenum C10, several transformants were obtained that produced 10-40% higher levels of cephalosporin. All selected transformants contained at least two or three copies of the mecB gene as shown by Southern hybridization with a probe internal to mecB. Two of these transformants, A. chrysogenum T27 and A. chrysogenum T58, showed 4- to 10-fold higher cystathionine-gamma -lyase activity than the control strain. Northern hybridizations indicated that the levels of the two mecB transcripts of 1.7 and 1.5 kb were greatly increased in transformants T27 and T58. Fermentor studies using controlled conditions confirmed that transformant T27 was a cephalosporin overproducer, reaching titers of nearly 2000 mug/ml of cephalosporin in Shen-defined medium that correlated with two- to fourfold higher cystathionine-gamma -lyase levels than in the control strain. Transformant T58 containing five- to sixfold higher levels of cystathionine-gamma -lyase in fermentor cultures showed a reduced growth, rate and a slow cephalosporin accumulation rate. In conclusion, moderately increased levels of cystathionine-gamma -lyase stimulated cephalosporin production but very high levels of this enzyme were deleterious for growth and cephalosporin biosynthesis.