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

L-cysteine is a precursor of the penicillin, cephalosporin and cephamycin f amilies of beta -lactam antibiotics. Cystathionine-gamma -lyase (encoded by the mecB gene), an enzyme that splits cystathionine releasing: cysteine, i s required for high-level cephalosporin production in methionine-supplement ed 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 c opies of the mecB gene as shown by Southern hybridization with a probe inte rnal to mecB. Two of these transformants, A. chrysogenum T27 and A. chrysog enum T58, showed 4- to 10-fold higher cystathionine-gamma -lyase activity t han the control strain. Northern hybridizations indicated that the levels o f the two mecB transcripts of 1.7 and 1.5 kb were greatly increased in tran sformants T27 and T58. Fermentor studies using controlled conditions confir med that transformant T27 was a cephalosporin overproducer, reaching titers of nearly 2000 mug/ml of cephalosporin in Shen-defined medium that correla ted with two- to fourfold higher cystathionine-gamma -lyase levels than in the control strain. Transformant T58 containing five- to sixfold higher lev els of cystathionine-gamma -lyase in fermentor cultures showed a reduced gr owth, rate and a slow cephalosporin accumulation rate. In conclusion, moder ately increased levels of cystathionine-gamma -lyase stimulated cephalospor in production but very high levels of this enzyme were deleterious for grow th and cephalosporin biosynthesis.