Evidence has been obtained for the presence of the small 22 kDa GTP-binding Rho protein in dormant spores of Phycomyces blakesleeanus. lmmunoblotting with a polyclonal antibody against RhoA revealed a soluble and membraneassociated 22 kDa protein. When [32P]ADP-ribosylated by Clostridium botulinum C3 exotoxin the protein had a pl of 5-7, a value similar to that reported for other RhoA proteins. The 22 kDa protein was expressed a t all stages of growth investigated, but radiolabelling of the [32P]ADP-ribosylated protein increased when tube-formation occurred and decreased as the hyphae branched. Localization of RhoA during spore germination studied by immunofluorescence microscopy revealed the presence of RhoA in the cell membrane of the spore. When the spore started to swell, RhoA was observed as patches in the cell membrane which become concentrated in the neck region of the site of the protuberation tube, but this protein was never observed a t the point of growth of the hyphal tip. The above results suggest that RhoA associated with one or more membrane proteins could participate in the molecular mechanism involved in maintaining cell integrity during the extrusion of the germ-tube of P. blakesleeanus.

We isolated three nikkomycin-resistant mutants of the dimorphic fungus M. rouxii which were physiologically characterized regarding their response to yeast-phase inducing conditions and their sensitivity to bacilysin. Mutant strains G21 and G23, showed a qualitatively normal, though delayed, dimorphic transition and partial cross-resistance to bacilysin. Mutant strain G27 showed an altered dimorphism, producing a high proportion (50%) of hyphal cells, and a wild-type sensitivity to bacilysin. Cell-free extracts from this mutant exhibited an activity of both basal and protease-activated chitin synthetase which was overexpressed as compared with the parental strain and mutants G21 and G23. Results are discussed in terms of the different genetic background of the mutants.