PhycomycesMADB interacts with MADA to form the primary photoreceptor complex for fungal phototropism

Abstract

The fungusPhycomyces blakesleeanusreacts to environmental signals, including light, gravity, touch, and the presence of nearby objects, by changing the speed and direction of growth of its fruiting body (sporangiophore). Phototropism, growth toward light, shares many features in fungi and plants but the molecular mechanisms remain to be fully elucidated.Phycomycesmutants with altered phototropism were isolated ≈40 years ago and found to have mutations in themadgenes. All of the responses to light inPhycomycesrequire the products of themadAandmadBgenes. We showed thatmadAencodes a protein similar to theNeurosporablue-light photoreceptor, zinc-finger protein WC-1. We show here thatmadBencodes a protein similar to theNeurosporazinc-finger protein WC-2. MADA and MADB interact to form a complex in yeast 2-hybrid assays and when coexpressed inE. coli, providing evidence that phototropism and other responses to light are mediated by a photoresponsive transcription factor complex. ThePhycomycesgenome contains 3 genes similar towc-1, and 4 genes similar towc-2, many of which are regulated by light in amadAormadBdependent manner. We did not detect any interactions between additional WC proteins in yeast 2-hybrid assays, which suggest that MADA and MADB form the major photoreceptor complex inPhycomyces. However, the presence of multiplewcgenes inPhycomycesmay enable perception across a broad range of light intensities, and may provide specialized photoreceptors for distinct photoresponses.