MUTANTS WITH GENERAL GROWTH-RATE ADVANTAGES ARE THE PREDOMINANT MORPHOLOGICAL MUTANTS TO BE ISOLATED FROM THE QUORN(R) PRODUCTION PLANT

Sixteen highly branched (colonial) mutants of Fusarium graminearum A3/ 5 were isolated at the end of 11 Quorn(R) myco-protein production ferm entations. These ranged from the highly branched C134-3 to the sparsel y branched C145, which was similar to A3/5 in liquid culture. Although allelic complementation was observed to occur between some of the mut ants, heterokaryon analysis revealed that all the highly branched muta nts belonged to a single complementation group. Mixed cultures of A3/5 and six of the colonial mutants were grown in glucose-, ammonium-, ma gnesium- and sulphate-limited chemostat culture. One mutant (C153) had a selective advantage over A3/5 in all nutrient limitations tested; l our mutants (C134-1, C134-3, C137-1, and C135) had a selective advanta ge over A3/5 in all except one nutrient limitation; and one mutant (C1 39) had a selective advantage over A3/5 in magnesium-limited chemostat culture only. Four mutants (C134-3, C139-1, C153 and C135) had a sele ctive advantage over A3/5 when the dilution rate was increased above t he critical dilution rate (0.22 h(-1)) and nutrients were present in e xcess. These results suggest that the growth conditions in the Quorn(R ) production plant (which is operated as a glucose-stat) favour mutati ons that confer general growth rate advantages, but these mutations ma y result in growth disadvantages when nutrient limitations are imposed . As all of the mutations appear to occur in one gene or gene cluster, the differing patterns of selective advantage/disadvantage for the si x mutants studied suggests that regulatory genes may also be involved or that different sites of mutation can lead to a variety of pleiotrop ic effects.