DISTURBED ASSEMBLY OF THE RESPIRATORY-CHAIN NADH - UBIQUINONE REDUCTASE (COMPLEX-I) IN CITRIC-ACID-ACCUMULATING ASPERGILLUS-NIGER STRAIN B-60

Previously we reported the selective loss of respiratory chain NADH:ubiquinone reductase (complex I) in the Aspergillus niger mutant B 60 during fermentation under citric-acid-accumulating conditions. The molecular basis underlying this complex I loss has been investigated. Complex I was isolated from A. niger wild-type and mutant B 60. No obvious differences in polypeptide composition and kinetic parameters were found between the two preparations. The mutant complex I, however, was very fragile and readily dissociated into subcomplexes when treated with salt. For studying the assembly of complex I, cells were labelled with [S-35]methionine and the flux of radioactivity was followed through immunoprecipitable intermediates into the holo-complex. In the mutant, but not in the wild-type, large quantities of labelled complex I polypeptides were observed to accumulate transiently in an intermediate subcomplex. The possibility is discussed that in mutant B 60 the assembly of complex I is disturbed, leading to an unstable complex I.