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Journal of Bacteriology, October 1999, p. 6339-6346, Vol. 181, No. 20
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Ras Signaling Is Required for Serum-Induced Hyphal Differentiation in Candida albicans

Qinghua Feng, Eric Summers,dagger Bing Guo, and Gerald Fink*

Whitehead Institute for Biomedical Research/Massachusetts Institute of Technology, Cambridge, Massachusetts 02142

Received 11 June 1999/Accepted 3 August 1999

Serum induces Candida albicans to make a rapid morphological change from the yeast cell form to hyphae. Contrary to the previous reports, we found that serum albumin does not play a critical role in this morphological change. Instead, a filtrate (molecular mass, <1 kDa) devoid of serum albumin induces hyphae. To study genes controlling this response, we have isolated the RAS1 gene from C. albicans by complementation. The Candida Ras1 protein, like Ras1 and Ras2 of Saccharomyces cerevisiae, has a long C-terminal extension. Although RAS1 appears to be the only RAS gene present in the C. albicans genome, strains homozygous for a deletion of RAS1 (ras1-2/ras1-3) are viable. The Candida ras1-2/ras1-3 mutant fails to form germ tubes and hyphae in response to serum or to a serum filtrate but does form pseudohyphae. Moreover, strains expressing the dominant active RAS1V13 allele manifest enhanced hyphal growth, whereas those expressing a dominant negative RAS1A16 allele show reduced hyphal growth. These data show that low-molecular-weight molecules in serum induce hyphal differentiation in C. albicans through a Ras-mediated signal transduction pathway.

* Corresponding author. Mailing address: Whitehead Institute for Biomedical Research, Nine Cambridge Center, Cambridge, MA 02142. Phone: (617) 258-5215. Fax: (617) 258-9872. E-mail: gfink{at}wi.mit.edu.

dagger Present address: Microbia, Inc., Cambridge, MA 02139.

Journal of Bacteriology, October 1999, p. 6339-6346, Vol. 181, No. 20
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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