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Antimicrob. Agents Chemother., 11 1997, 2471-2479, Vol 41, No. 11
CM Douglas, JA D'Ippolito, GJ Shei, M Meinz, J Onishi, JA Marrinan, W Li, GK Abruzzo, A Flattery, K Bartizal, A Mitchell and MB Kurtz
Pneumocandins and echinocandins are fungicidal antibiotics, currently in
clinical development, that inhibit 1,3-beta-D-glucan synthase (GS) in
several human fungal pathogens. We have identified a gene from the diploid
organism Candida albicans that encodes a target of these inhibitors. A
2.1-kb portion of this gene, designated CaFKS1, has significant homology to
the Saccharomyces cerevisiae FKS1 and FKS2 genes, which encode partially
functionally redundant subunits of GS. To evaluate the role of CaFkslp in
susceptibility to echinocandins, we disrupted CaFKS1 on one homolog each of
the spontaneous pneumocandin- resistant C. albicans mutants CAI4R1, NR2,
NR3, and NR4. These mutants had been selected previously on agar plates
containing the pneumocandin L-733,560. The clones derived from this
transformation were either resistant (Ech[r]) or fully sensitive (Ech[s])
to inhibition by L- 733,560 in both liquid broth microdilution and in vitro
GS assays. The site of plasmid insertion in the transformants was mapped by
Southern blot analysis, using restriction site polymorphisms in the CaFKS1
gene to distinguish between the two alleles (designated CaFKS1h and
CaFKS1b). For strains CAI4R1 and NR2, the CaFKS1b allele was disrupted in
each Ech(r) transformant; for strain NR4, CaFKS1h was disrupted in each
Ech(r) transformant. We conclude that (i) strains CAI4R1, NR2, and NR4 are
heterozygous for a dominant or semidominant pneumocandin resistance
mutation at CaFKS1, (ii) drug resistance mutations can occur in either
CaFKS1 allele, and (iii) CaFks1p is a target of the echinocandins. For
transformants of strain NR3, all the clones we analyzed were uniformly
Ech(r), and only the CaFKS1h allele, either in disrupted or wild-type form,
was detected on genomic Southern blots. We believe gene conversion at the
CaFKS1 locus may have produced two Cafks1h alleles that each contain an
Ech(r) mutation. Transformants derived from the mutants were analyzed for
susceptibility to pneumocandin treatment in a mouse model of disseminated
candidiasis. Strains heterozygous for the resistant allele (i.e., C.
albicans CAI4R1, NR2, and NR4) were moderately resistant to treatment,
while strains without a functional Ech(s) allele (i.e., strain NR3 and
derivatives of strain CAI4R1 with the disruption plasmid integrated in the
Ech[s] allele) displayed strong in vivo echinocandin resistance. Finally,
we were unable to inactivate both alleles at CaFKS1 by two- step
integrative disruption, suggesting that CaFks1p is likely to be an
essential protein in C. albicans.
Copyright © 1997 by the American Society for Microbiology. All rights reserved.
Identification of the FKS1 gene of Candida albicans as the essential target of 1,3-beta-D-glucan synthase inhibitors
Department of Biochemistry, Merck Research Laboratories, Rahway, New Jersey 07065, USA.
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