Sterol C-14 reductase catalyses the reduction of the Delta(14,15) bond in intermediates in the sterol biosynthesis pathway using NADPH as a cofactor. We have undertaken a systematic site-directed mutational analysis of all the conserved charged and potentially proton-donating residues of the sterol C-14 reductase from Neurospora crassa. The effect of each mutation was determined using an in vivo assay based on the complementation of the corresponding N. crassa mutant ( erg-3). The non-complementing mutations were also tested in the erg24 mutant of Saccharomyces cervisiae. The results are discussed with reference to the predicted topology of the enzyme and to its proposed catalytic mechanism, which involves addition of a proton from an appropriately positioned charged or polar residue to the substrate double bond, followed by addition of hydride ion from NADPH.

Four insertional or quasiterminal translocations (T) were recently introgressed fromNeurospora crassa into N. tetrasperma. Crosses of two of the resulting T Nt strains with N. tetrasperma N strains (N = normal sequence) produced more Dp than T and N homokaryotic progeny, although [T + N] and [Dp + Df] heterokaryotic progeny were made in roughly equal numbers. The T, N, and [T + N] progeny are derived from alternate segregation (ALT), whereas adjacent-1 segregation (ADJ) generates the Dp, Df, and [Dp + Df] types. Differential recovery of homokaryotic products from ALT and ADJ represents a novel and unprecedented type of meiotic drive. This drive contributed to our inability to introgress a larger insertions translocation, T(VR>VIL)UK3-41, into N. tetrasperma. We suggest that one or more Bateson-Dobzhansky-Muller type incompatibility between N. crassa and N. tetrasperma genes in the T Nt x N crosses might cause an insufficiency for a product required for ascospore maturation. Since the Df type is inviable, only four ascospores (Dp or [Dp + Df] types) share this limited resource in [Dp + Df] asci, whereas four to eight ascospores compete for it in [T + N] asci. This increases the chance that in asci with >4 ascospores none properly matures, and results in Dp progeny out-numbering T and N types.