A SURFACE-ACTIVE PROTEIN INVOLVED IN AERIAL HYPHAE FORMATION IN THE FILAMENTOUS FUNGUS SCHIZOPHILLUM COMMUNE RESTORES THE CAPACITY OF A BALD MUTANT OF THE FILAMENTOUS BACTERIUM STREPTOMYCES-COELICOLOR TO ERECTAERIAL STRUCTURES

The filamentous bacterium Streptomyces coelicolor undergoes a complex process of morphological differentiation involving the formation of a dense lawn of aerial hyphae that grow away from the colony surface int o the air to form an aerial mycelium, Bald mutants of S, coelicolor, w hich are blocked in aerial mycelium formation, regain the capacity to erect aerial structures when exposed to a small hydrophobic protein ca lled SapB, whose synthesis is temporally and spatially correlated with morphological differentiation. We now report that SapB is a surfactan t that is capable of reducing the surface tension of water from 72 mJ m(-2) to 30 mJ m(-2) at a concentration of 50 mu g ml(-1) We also repo rt that SapB, like the surface-active peptide streptofactin produced b y the species S. tendae, was capable of restoring the capacity of bald mutants of S. tendae to erect aerial structures. Strikingly, a member (SC3) of the hydrophobin family of fungal proteins involved in the er ection of aerial hyphae in the filamentous fungus Schizophyllum commun e was also capable of restoring the capacity of S, coelicolor and S, t endae bald mutants to erect aerial structures. SC3 is unrelated in str ucture to SapB and streptofactin but, like the streptomycetes proteins , the fungal protein is a surface active agent. Scanning electron micr oscopy revealed that aerial structures produced in response to both th e bacterial or the fungal proteins were undifferentiated vegetative hy phae that had grown away from the colony surface but had not commenced the process of spore formation, We conclude that the production of Sa pB and streptofactin at the start of morphological differentiation con tributes to the erection of aerial hyphae by decreasing the surface te nsion at the colony surface but that subsequent morphogenesis requires additional developmentally regulated events under the control of bald genes.