Partial Enzyme Aggregates Formed by Pleiotropic Mutants in the arom Gene Cluster of Neurospora crassa

Molecular weights of enzymically active arom aggregates produced by pleiotropic and various arom-1 mutants of Neurospora crassa have been estimated by sucrose density gradient centrifugation. In contrast to most single-gene mutants (which produce intact arom multienzyme aggregates of normal molecular weight- about 230,000), pleiotropic mutants that lack two or more of the five enzyme activities in the arom aggregate produce partial arom aggregates of molecular weights ranging from about 60,000 to about 85,000. In addition, certain arom-1 mutants are pleiotropic in producing arom aggregates of about half the normal molecular weight. Experiments with heterocaryons, as well as with mixtures of extracts from various mutants, have provided evidence concerning the presence or absence of interactions (as detectable molecular hybridization) between different arom aggregates. An evaluation has been made of the relation between the genetic location of particular mutants within the arom gene cluster and the size and enzymic content of the arom aggregates they produce. Interpretations concerning the molecular organization of the arom multienzyme aggregate in N. crassa are presented.