The production kinetics of a transformed strain of Penicillium chrysogenum expressing the expandase gene from Streptomyces clavuligerus was investigated in chemostat cultivations. The recombinant strain produces adipoyl-7-aminodeacetoxycephalosporanic acid (ad-7-ADCA) as the major product; however, during the cultivations, the appearance of a major unknown and poorly secreted product was observed. Investigations using high-performance liquid chromatography (HPLC) and liquid chromatography-mass spectroscopy (LC-MS) showed that this byproduct has a six-membered dihydrothiazine ring, which is characteristic for cephalosporins. The byproduct may be formed via isopenicillin N by as-yet unknown mechanisms, but involving expandase. It is likely that the unknown compound (UC) is deacetoxycephalosporin C (DAOC). Investigation of the instability of the various beta-lactams produced showed higher instability for compounds with a five-membered thiazolidine ring than those with a six-membered dihydrothiazine ring. Furthermore, secretion of products and by-products was shown to be quite different. The productivity was studied as a function of the dilution rate in the range 0.015 to 0.090 h(-1). The specific productivity of total beta-lactams was compared with that of the penicillin-G-producing host strain, and it was found to be lower at dilution rates of <0.06 h(-1). Quantification of the fluxes through the pathway leading to ad-7-ADCA showed a decrease in flux toward ad-7-ADCA, and an increase in flux toward UC as the dilution rate increased. Northern analysis of the biosynthetic genes showed that expression of the enzymes involved in the ad-7-ADCA pathway decreased as the dilution rate increased. (C) 2003 Wiley Periodicals, Inc.