335Regulation of the cross-pathway type was found in Aspergillus nidulans. Ornithine carbamoyltransferase, a biosynthetic enzyme of the arginine pathway, is derepressed as a result of deprivation not only of arginine but also of histidine, proline and tryptophan. Crosspathway regulation operates in the mutant suD19 in which the ornithine carbamoyltransferase level is insensitive to arginine. The spectrum of the amino acids involved in this type of regulation seems to be different from those described for other fungi.The phenomenon of general or cross-pathway regulation of the biosynthesis of amino acids has been described for the fungus Neurospord crassa (Carsiotis et al., 1974) and for yeasts (Schurch et ul., 1974;Wolfner et al., 1975). Starvation for a given amino acid results in simultaneous derepression not only of its own biosynthetic pathway, but also of several unrelated pathways. In N . crassa, starvation for either histidine, arginine or tryptophan results in a simultaneous derepression of biosynthetic enzymes for all of them. In Saccharomyces cerevisiae, the enzymes of the biosynthetic pathways for arginine, lysine, leucine, isoleucine/valine, histidine, tryptophan and probably some others are derepressed during limitation of any of these amino acids.We have shown previously that in Aspergillus nidulans ornithine carbamoyltransferase (OCT), an enzyme of the arginine biosynthetic pathway, is derepressed under conditions of arginine limitation (Cybis et al., 1970). OCT appears to be under the control of the same regulatory gene SUD as the arginine inducible enzymes of arginine catabolism (Cybis et al., 1972). The results presented here indicate the existence of a cross-pathway control of OCT synthesis.
M E T H O D SStrains. These were all from the collection of the Department of Genetics, University of Warsaw. The symbols arg, bi, his, lu, lys, paba, pro and pyro denote auxotrophy for arginine, biotin, histidine, leucine, lysine, p-aminobenzoic acid, proline and pyridoxine, respectively ; w and y indicate white and yellow conidia, respectively .Media. The minimal medium (MM) of Cove (1966) was used. Methods of culfure, harvesting of mycelia and preparation of extracfs. These were as described previously (Cybis et al., 1970). Starvation experiments were performed in three ways. (a) About 20 h-old mycelium grown on medium with all the required supplements was collected, washed with MM, transferred to medium without amino acid supplements and harvested after 8 h. (6) An amino acid-requiring non-leaky auxotroph was cultured on the appropriate amino acid at a growth-rate limiting concentration. The growth with the amino acid at different concentrations had previously been established by following the increase in mycelial dry weight. (e) The leaky auxotroph was grown on medium without amino acid supplements.Enzyme assaj'. Ornithine carbamoyltransferase (OCT; carbamoylphosphate: L-ornithine carbamoyltransferase; EC 2.1 . 3 . 3 ) activity was determined as described previously (Cybis et ul., 1970).
