Nutrient‐dependent selection of morphological mutants ofFusarium graminearumA3/5 isolated from long‐term continuous flow cultures

Abstract

Highly branched (colonial) mutants (MC1-1-, CC1-1, and C106) of Fusarium graminearum A3/5 were each grown with the parental strain (A3/5) in continuous flow cultures at high and low dilution rates using a variety of nutrient limitations. MC1-1 replaced A3/5 in all nutrient-limited cultures tested (glucose-, Mg(2+)-, ammonium-, and sulphate-limited cultures), suggesting that it has a higher maximum specific growh rate than A3/5. Compared with A3/5, C106 was positively selected for in Mg(2+)-limited cultures and its selection coefficient was higher at low than at high dilution rates, suggesting that, compared with A3/5, it has a reduced saturation constant (K(s)) for Mg(2+). However, in batch culture, C106 and A3/5 had the same (15 microM) appaent K(s) value for Mg(2+). C106 was replaced (negative selection coefficient) by A3/5 in gluose-, ammonium-, and phsophate-limited continuous flow cultures, but was neither at an advantage nor a disadvantage (i.e., it behaved as a neutral mutation) in sulphate-limited cultures. CC1-1 replaced A3/5 when they were grown together in glucose-, maltose-, or ribose-limited continuous flow cultures, but not in fructose-, xylose-, ammonium-, or phsophate-limited cultures. Because A3/5 and CC1-1 had similar K(m) values (30 microM) for glucose, and because the selective advantage of CC1-1 was maintained in maltose-limited cultures (maltose was not hydrolyzed extracellularly), it was concluded that the selective advantage of CC1-1 did not result from it having a lower K(s) for glucose than the parental strain. Rather, the data suggested that the activity of phosphoketopentoepimerase may be altered by the CC1-1 mutation.

Effect of branch frequency inAspergillus oryzae on protein secretion and culture viscosity

“…It is known that increased branching is part of a pleiotropic response to altered enzyme activity in glucose-6-P dehydrogenase and phosphoglucomutase mutants of Neurospora crassa Tatum, 1966, 1967) and pleiotropic effects are also implicated in the highly branched mutants isolated from F. venenatum fermentations (Wiebe et al, 1992). However, improved culture rheology, rather than improved secretion or production, Figure 6.…”

Evolution of Fusarium graminearum A3/5 grown in a glucose-limited chemostat culture at a slow dilution rate

“…3). However, as the selection coefficient of A28-S relative to A3/5 actually became (Wiebe et al, 1992).…”

“…Thus, increased hyphal branching does not appear to be associated with selective advantages of mutants selected at low dilution rates in glucose-limited chemostat cultures of F. graminearam. Wiebe et al (1992) showed that a mutation in an enzyme of the pentose phosphate pathway in F. graminearam A3/5 resulted in a colonial mutant with a selective advantage in glucose-limited cultures at high dilution rates. The present results suggest that strain A28-S had an altered carbon metabolism as well as an improved glucose transport system.…”

“…K , values for glucose uptake were measured (in duplicate) as described by Wiebe et al (1992). Uptake was followed for 150 s after the addition of 10-50 pM (final concentration) glucose [O-5 pCi (1 8.5 kBq) [14C]glucose pM-l, specific activity 290 mCi (10.7 MBq) mmol-', Amersham].…”

“…Using chemostat cultures grown at low dilution rates, Novick & Szilard (1 950b) isolated a mutant of Escbericbia coli with improved uptake of tryptophan, and Adams et al (1985) isolated mutants of Saccharomjces cerevisiae with improved glucose uptake. However, apart from a mutant of F. gramineartrm with a possible improvement in magnesium uptake (Wiebe et al, 1992), K, mutants of filamentous fungi have not been isolated from chemostat cultures. Similary, using chemostat cultures at high dilution rates, Helling e t al.…”