Members of the Penicillium chrysogenum velvet complex play functionally opposing roles in the regulation of penicillin biosynthesis and conidiation.

A velvet multisubunit complex was recently detected in the filamentous fungus Penicillium chrysogenum, the major industrial producer of the β-lactam antibiotic penicillin. Core components of this complex are P. chrysogenum VelA (PcVelA) and PcLaeA, which regulate secondary metabolite production, hyphal morphology, conidiation, and pellet formation. Here we describe the characterization of PcVelB, PcVelC, and PcVosA as novel subunits of this velvet complex. Using yeast two-hybrid analysis and bimolecular fluorescence complementation (BiFC), we demonstrate that all velvet proteins are part of an interaction network. Functional analyses using single- and double-knockout strains clearly indicate that velvet subunits have opposing roles in the regulation of penicillin biosynthesis and light-dependent conidiation. PcVelC, together with PcVelA and PcLaeA, activates penicillin biosynthesis, while PcVelB represses this process. In contrast, PcVelB and PcVosA promote conidiation, while PcVelC has an inhibitory effect. Our genetic analyses further show that light-dependent spore formation depends not only on PcVelA but also on PcVelB and PcVosA. The results provided here contribute to our fundamental understanding of the function of velvet subunits as part of a regulatory network mediating signals responsible for morphology and secondary metabolism and will be instrumental in generating mutants with newly derived properties that are relevant to strain improvement programs.