Molecular characterization of the Acremonium chrysogenum cefG gene product: the native deacetylcephalosporin C acetyltransferase is not processed into subunits

Constructions starting at each of the three in-frame ATG codons of the Acre monium chrysogenum cefG gene (Met(1), Met(46) and Met(60)) were expressed i n Escherichia coli, obtaining proteins of 49, 44 and 43 kDa, respectively. All three proteins showed deacetylcephalosporin C (DAC) acetyltransferase a ctivity. The native A. chrysogenum DAC acetyltransferase was purified to el ectrophoretic homogeneity by immunoaffinity chromatography, It showed a mol ecular mass of 50 kDa by filtration in calibrated Sephadex G-75 SF or Super ose 12 (FPLC) columns. The N-terminal end of the pure DAC acetyltransferase was Met-Leu-Pro-Ser-Ala-Gln-Val-Ala-Arg-Leu, which matched perfectly the d educed amino acid sequence starting at Met(1). The putative alpha- and beta -subunits of DAC acetyltransferase were also obtained in E. coli but showed no enzymic activity either separately or in combination. Immunoblotting (W estern) analysis revealed that the 50 kDa DAC acetyltransferase showed high protein levels in A. chrysogenum cultures at 72 and 96 h and decreased sha rply thereafter, but in all cases no detectable processing of the enzyme in to subunits was found. Three different A, chrysogenum strains (including th e wild-type Brotzu strain and two high-cephalosporin-producing mutants) sho wed the same unprocessed 50 kDa DAC acetyltransferase, The nonproducer muta nt ATCC 20371 showed no DAC acetyltransferase protein band but formed a nor mal transcript of 1,4 kb.