IDENTIFICATION AND CHARACTERIZATION OF ASPERGILLUS-NIDULANS MUTANTS DEFECTIVE IN CYTOKINESIS

Filamentous fungi undergo cytokinesis by forming crosswalls termed sep ta. Here, we describe the genetic and physiological controls governing septation in Aspergillus nidulans. Germinating conidia do not form se pta until the completion of their third nuclear division. The first se ptum is invariantly positioned at the basal end of the germ tube. Bloc k-and-release experiments of nuclear division with benomyl or hydroxyu rea, and analysis of various nuclear division mutants demonstrated tha t septum formation is dependent upon the third mitotic division. Block -and-release experiments with cytochalasin A and the localization of a ctin in germlings by indirect immunofluorescence showed that actin par ticipated in septum formation. In addition to being concentrated at th e growing hyphal tips, a band of actin was also apparent at the site o f septum formation. Previous genetic analysis in A, nidulans identifie d four genes involved in septation (sepA-D). We have screened a new co llection of temperature sensitive (ts) mutants of A. nidulans for stra ins that failed to form septa at the restrictive temperature but were able to complete early nuclear divisions. We identified five new genes designated sepE, G, H, I and J, along with one additional allele of a previously identified septation gene. On the basis of temperature shi ft experiments, nuclear counts and cell morphology, we sorted these cy tokinesis mutants into three phenotypic classes. Interestingly, one cl ass of mutants fails to form septa and fails to progress past the thir d nuclear division. This class of mutants suggests the existence of a regulatory mechanism in A, nidulans that ensures the continuation of n uclear division following the initiation of cytokinesis.