Targeted gene disruption efficiency in Acremonium chrysogenum was increased 10-fold by applying the double-marker enrichment technique to this filamentous fungus. Disruption of the mecB gene by the doublemarker technique was achieved in 5% of the transformants screened. Mutants T6 and T24, obtained by gene replacement, showed an inactive mecB gene by Southern blot analysis and no cystathionine-␥-lyase activity. These mutants exhibited lower cephalosporin production than that of the control strain, A. chrysogenum C10, in MDFA medium supplemented with methionine. However, there was no difference in cephalosporin production between parental strain A. chrysogenum C10 and the mutants T6 and T24 in Shen's defined fermentation medium (MDFA) without methionine. These results indicate that the supply of cysteine through the transsulfuration pathway is required for high-level cephalosporin biosynthesis but not for low-level production of this antibiotic in methionine-unsupplemented medium. Therefore, cysteine for cephalosporin biosynthesis in A. chrysogenum derives from the autotrophic (SH 2 ) and the reverse transsulfuration pathways. Levels of methionine induction of the cephalosporin biosynthesis gene pcbC were identical in the parental strain and the mecB mutants, indicating that the induction effect is not mediated by cystathionine-␥-lyase.
-Lactam biosynthesis inAcremonium chrysogenum (formerly known as Cephalosporium acremonium) begins with the nonribosomal condensation of the three precursor amino acids L-␣-aminoadipic acid, L-cysteine, and L-valine (1, 21, 23). There are two ways to synthesize cysteine in filamentous fungi (35,26). One pathway, the so-called autotrophic pathway, converts inorganic sulfur to cysteine via serine O-acetyltransferase and O-acetylserine sulfhydrilase (Fig. 1). In the second route, cysteine can be obtained via the reverse transsulfuration pathway, in which the sulfur atom of methionine is transferred to cysteine through S-adenosylmethionine, S-adenosylhomocysteine, homocysteine, and cystathionine as intermediates.The predominant route of the cysteine supply for -lactam biosynthesis depends on the producer microorganism (6). Cysteine for penicillin biosynthesis in Penicillium chrysogenum is obtained mainly from sulfate reduction (31). In this microorganism, the transsulfuration pathway has been considered to have a minor role in penicillin biosynthesis. On the other hand, the sulfur atom for -lactam biosynthesis by A. chrysogenum is believed to derive preferentially from methionine via the reverse transsulfuration pathway (3, 24). However, there is no genetic evidence for, or against, this hypothesis.DL-Methionine is known to stimulate cephalosporin biosynthesis in A. chrysogenum (7, 16). For many years it was unclear whether the stimulatory effect of DL-methionine was due to a precursor effect (one providing cysteine) or to an inducing effect (8,20,23). Sawada and coworkers established that methionine increases isopenicillin N synthase, deacetoxycephalosporin C synthase (30), and ␣-...