Polarity-defective mutants of Aspergillus nidulans

We have identified two polarity-defective (pod) mutants in Aspergillus nidulans from a collection of heat-sensitive lethal mutants. At restrictive temperature, these mutants are capable of nuclear division but are unable to establish polar hyphal growth. We cloned the two pod genes by complementation of their heat-sensitive lethal pheno-types. The libraries used to clone the pod genes are under the control of the bidirectional niaD and niiA promoters. Complementation of the pod mutants is dependent on growth on inducing medium. We show that rescue of the heat-sensitive phenotype on inducing media is independent of the orientation of the gene relative to the niaD or niiA promoters, demonstrating that the intergenic region between the niaD and the niiA genes functions as an orientation-independent enhancer and repressor that is capable of functioning over long distances. The products of the podG and the podH genes were identified as homologues of the α subunit of yeast mitochondrial phenylalanyl-tRNA synthetase and transcription factor IIF interacting component of the CTD phosphatase. Neither of these gene products would have been predicted to produce a pod mutant phenotype based on studies of cellular polarity mutants in other organisms. The implications of these results are discussed.