Repellents have functionally replaced hydrophobins in mediating attachment to a hydrophobic surface and in formation of hydrophobic aerial hyphae in Ustilago maydis

1 Microbiology, Institute of Biomembranes, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands 2 MPI für Terrestrische Mikrobiologie, Karl-von-Frisch-Straße, 35043 Marburg, Germany 3 Institute für Genetik, Ludwig-Maximilians University, Maria-Ward-Straße 1a, 80638 Munich, Germany Correspondence Han A. B. Wösten h.a.b.wosten{at}bio.uu.nl Ustilago maydis contains one repellent and two class I hydrophobin genes in its genome. The repellent gene rep1 has been described previously. It encodes 11 secreted repellent peptides that result from the cleavage of a precursor protein at KEX2 recognition sites. The hydrophobin gene hum2 encodes a typical class I hydrophobin of 117 aa, while hum3 encodes a hydrophobin that is preceded by 17 repeat sequences. These repeats are separated, like the repellent peptides, by KEX2 recognition sites. Gene hum2 , but not hum3 , was shown to be expressed in a cross of two compatible wild-type strains, suggesting a role of the former hydrophobin gene in aerial hyphae formation. Indeed, aerial hyphae formation was reduced in a hum2 cross. However, the reduction in aerial hyphae formation was much more dramatic in the rep1 cross. Moreover, colonies of the rep1 cross were completely wettable, while surface hydrophobicity was unaffected and only slightly reduced in the hum2 and the hum2 hum3 cross, respectively. It was also shown that the repellents and not the hydrophobins are involved in attachment of hyphae to hydrophobic Teflon. Deleting either or both hydrophobin genes in the rep1 strains did not further affect aerial hyphae formation, surface hydrophobicity and attachment. From these data it is concluded that hydrophobins of U. maydis have been functionally replaced, at least partially, by repellents. These authors contributed equally to this work.