Mas5, a homologue of bacterial DnaJ, is indispensable for the host infection and environmental adaptation of a filamentous fungal insect pathogen

Summary Mas5, a yeast heat shock protein classified to the Hsp40 group, is homologous to bacterial archetype DnaJ but functionally unexplored in filamentous fungi. Here we identify a Mas5 homologue (46.86 kDa) in B eauveria bassiana and show its indispensability for host infection and environmental adaptation of the fungal insect pathogen. The deletion of mas5 caused severe defects in aerial conidiation, conidial germination and submerged blastospore production (mimic to host haemocoel). The deletion mutant lost 100% virulence to G alleria mellonella larvae through normal cuticular penetration (topical inoculation) and 50% through cuticle‐bypassing infection (intrahaemocoel injection). It formed no blastospore in vivo after inoculation or only a very few after injection. Its extracellular (cuticle degrading) enzymes and virulence‐relating Pr1 proteases were 62% and 32% less active respectively. It became more sensitive to high osmolarity, oxidation, cell‐wall perturbation, heat shock and UV ‐ B irradiation. These concurred with reduced contents of intracellular mannitol and trehalose, decreased activities of antioxidant enzymes, impaired cell walls and suppressed transcripts of stress‐responsive and virulence‐relating genes. All the changes were restored by targeted mas5 complementation. All together, Mas5 is indispensable for the in vitro and in vivo life cycle of B . bassiana by targeting many sets of enzymes/proteins at transcriptional and post‐transcriptional levels.