To increase the frequency of homologous recombination, we inactivated the KU80 homologue in Aspergillus fumigatus (named akuB ^KU80 ). Homologous integration reached about 80% for both calcineurin A ( calA ) and polyketide synthase pksP (alb1 ) genes in the akuB ^KU80 mutant to 3 and 5%, respectively, when using a wild-type A. fumigatus strain. Deletion of akuB ^KU80 had no influence on pathogenicity in a low-dose murine infection model.

Summary: The protein phosphatase calcineurin is an important mediator connecting calcium‐dependent signalling to various cellular responses in multiple organisms. In fungi calcineurin acts largely through regulating Crz1p‐like transcription factors. Here we characterize an Aspergillus fumigatus CRZ1 homologue, CrzA and demonstrate its mediation of cellular tolerance to increased concentrations of calcium and manganese. In addition to acute sensitivitiy to these ions, and decreased conidiation, the crzA null mutant suffers altered expression of calcium transporter mRNAs under high concentrations of calcium, and loss of virulence when compared with the corresponding complemented and wild‐type strains. We use multiple expression analyses to probe the transcriptional basis of A. fumigatus calcium tolerance identifying several genes having calA and/or crzA dependent mRNA accumulation patterns. We also demonstrate that contrary to previous findings, the gene encoding the Aspergillus nidulans calcineurin subunit homologue, cnaA, is not essential and that the cnaA deletion mutant shares the morphological phenotypes observed in the corresponding A. fumigatus mutant, ΔcalA. Exploiting the A. nidulans model system, we have linked calcineurin activity with asexual developmental induction, finding that CrzA supports appropriate developmental induction in a calcineurin and brlA‐dependent manner in both species.