Specialized Sugar Sensing in Diverse Fungi

Report

Specialized Sugar Sensing

in Diverse Fungi

Victoria Brown,

Jeffrey Sabina,

and Mark Johnston

Department of Genetics

Center for Genome Sciences

Washington University School of Medicine

St. Louis, MO 63108

USA

Summary

S. cerevisiae senses glucose and galactose differently.

Glucose is detected through sensors that reside in the

cellular plasma membrane. When activated, the sensors

initiate a signal-transduction cascade that ultimately inacti-

vates the Rgt1 transcriptional repressor by causing degrada-

tion of its corepressors Mth1 and Std1 [1, 2]. This results in

the expression of many HXT genes encoding glucose

transporters [3]. The ensuing ﬂood of glucose into the cell

activates Mig1, a transcriptional repressor that mediates

‘‘glucose repression’’ of many genes, including the GAL

genes; hence, glucose sensing hinders galactose utilization

[4–6]. Galactose is sensed in the cytoplasm via Gal3. Upon

binding galactose (and ATP), Gal3 sequesters the Gal80

protein, thereby emancipating the Gal4 transcriptional acti-

vator of the GAL genes [7]. Gal4 also activates expression

of MTH1, encoding a corepressor critical for Rgt1 function

[8]. Thus, galactose inhibits glucose assimilation by encour-

aging repression of HXT genes. C. albicans senses glucose

similarly to S. cerevisiae but does not sense galactose

through Gal3-Gal80-Gal4 [9]. Its genome harbors no GAL80

ortholog, and the severely truncated CaGal4 does not

regulate CaGAL genes [9, 10]. We present evidence that

C. albicans senses galactose with its Hgt4 glucose sensor,

a capability that is enabled by transcriptional ‘‘rewiring’’ of

its sugar-sensing signal-transduction pathways. We suggest

that galactose sensing through Hgt4 is ancestral in fungi.

Results and Discussion

Hgt4 Affects Cell Growth and Filamentation on Galactose

C. albicans Dhgt4 mutants cannot grow on glucose in the pres-

ence of the respiration inhibitor antimycin A [11], which forces

cells to ferment glucose and demands a high rate of glucose

inﬂux. Because galactose and glucose are structurally similar,

it seemed plausible that the Hgt4 glucose sensor might sense

galactose (Figure 1). Indeed, Dhgt4 cells have a marked growth

defect on galactose with antimycin A (Figure S1A, available

online), suggesting that Hgt4 is required for galactose utiliza-

tion (see Table S3 for strains used in this study). Galactose

induces robust ﬁlamentation (yeast-to-hyphal morphogen-

esis) of C. albicans cells, and the Dhgt4 cells are also defective

in this response (Figure S1B). Thus, in the absence of Hgt4,

C. albicans cells display growth and morphological defects

in galactose.

Galactose and Glucose Induce Expression

of the Same Genes

Expression

of 49 genes increased by R 2-fold (Table 1, Groups

I–III) in response to 2% galactose (compared to glycerol). Most

of these galactose-induced genes (40, or 82%) are also signif-

icantly induced by 2% glucose (Table 1, Group I). Six of the

nine genes that were not induced by 2% glucose are in fact

induced by low glucose levels (<0.2%) but have been shown

to be repressed in cells exposed to the high level of glucose

used here (Table 1, Group II) [11–15]. Only three genes are

modestly induced by galactose but not induced by glucose

(Table 1, Group III). Therefore, 94% (46/49) of the characterized

genes that are induced in response to galactose are also

induced in response to low or high levels of glucose.

Hgt4 Affects the Transcriptional Response to Galactose

Expression of ﬁve of the top genes listed in Table 1 (Group I)

was reexamined by RT-PCR analysis. In cells grown on glyc-

erol, these genes are either not expressed (HGT7, QDR1,

AOX2) or expressed at low levels (CMK1, HXK2), and all ﬁve

are induced in response to galactose in an Hgt4-dependent

manner (Figure 2). HGT12, encoding a glucose transporter

related to Hgt4 [11, 16], does not affect the expression of

these genes. Induction of GAL1 expression by galactose is

signiﬁcantly diminished in the Dhgt4 mutant (Figure S2),

consistent with the previous observation that the Hgt4 signal

increases GAL1 and GAL7 expression 2-fold [11, 12]. Galac-

tose still induces GAL1 expression in Dhgt4 cells, indicating

that another signaling pathway contributes to GAL1 expres-

sion, possibly by acting upon Cph1 (a C. albicans homolog

of S. cerevisiae Ste12) [9].

The CaHGT7 gene, encoding a hexose transporter, is highly

induced—over 30-fold—by both galactose and glucose (Table

1). HGT7 expression is activated by low levels (0.04%) of

glucose, fructose, or mannose (Figure 3A, top) and by a high

level (1.6%) of galactose (Figure 3A, bottom).

HGT7 expression

response

to sugars is entirely dependent on HGT4

(Figure 3B), and Hgt4 mediates the dose-dependent galactose

induction of HGT7 expression at concentrations as low as

0.6% (Figure S3).

Galactose-Induced Genes Have Rgt1-Binding Sites

Of the 50 genes most highly induced by galactose, 34 (68%)

contain at least one consensus Rgt1 DNA-binding motif (5

CGGANNA-3

) within 1 kb upstream of the translational start

codon (Table S1). This is a signiﬁcant enrichment (p <

)—only 46% of promoters genome-wide harbor an Rgt1

motif—that is similar to the enrichment of consensus Rgt1-

binding sites upstream of genes regulated by glucose via

Hgt4 and CaRgt1 (66%, p < 10

) (see Supplemental Experi-

mental Procedures for statistical methods). The CaCph1 tran-

scription factor has also been implicated in the expression of

the CaGAL genes in response to galactose [9], but its

binding-site is not enriched in other galactose-induced genes

(Table S1). The promoters of the GAL1-10 and GAL7 genes

(encoding the enzymes for galactose metabolism) each

contain both a perfect Cph1 response element (5

-TGTAAC

GTTACA-3

) [9] and two Rgt1 recognition-sequence motifs,*Correspondence: vbrownk@wustl.edu

brought to you by COREView metadata, citation and similar papers at core.ac.uk

provided by Elsevier - Publisher Connector

consistent with the idea that Rgt1 and Cph1 coordinately regu-

late these genes in response to galactose.

Hgt4 Senses Galactose in S. cerevisiae

In S. cerevisiae, HXT genes are induced by glucose, fructose,

and mannose but not by galactose, ostensibly because Snf3

and Rgt2 do not bind galactose [17]. If Hgt4 binds galactose,

then expressing it in S. cerevisiae should cause galactose

induction of HXT genes. The HGT4 sugar-binding domain

(codon optimized) was expressed in S. cerevisiae from the

RGT2 promoter (see Supplemental Experimental Procedures).

Because the C-terminal cytoplasmic tails of the glucose

sensors have diverged almost completely in the w200 million

years since the C. albicans and S. cerevisiae lineages

diverged, the Hgt4 sugar-binding domain was fused to the

Rgt2 tail to enable coupling of the sensor to the S. cerevisiae

signal-transduction pathway (Figure S4) [11]. Exchanging the

intracellular signaling tails of glucose sensors does not affect

their response to glucose (V.B., unpublished data; V. Brachet,

unpublished data), so we are conﬁdent that the Hgt4-Rgt2

chimera retains the sugar-sensing speciﬁcity of Hgt4. In

S. cerevisiae cells expressing the Hgt4 chimera, HXT1 is not

induced by galactose (Figure 4A, black bars; Figure S5, third

row). However, galactose induces MTH1 expression in

S. cerevisiae via Gal4 [8], and the resulting increase in Mth1

levels would be expected to reinforce Rgt1-mediated repres-

sion of HXT1, effectively masking any galactose signal gener-

ated by Hgt4 in S. cerevisiae. Deleting ScGAL4 eliminates this

control element and reveals robust activation of the HXT1-lacZ

reporter in response to galactose in cells expressing the Hgt4

chimera (Figure 4A, blue bars; Figure S5, bottom row) [18].In

contrast, neither Rgt2 nor Snf3 (which are present in these

strains) respond to galactose (indicated by cells with the

vector control; Figure 4A, gray bars; Figure S5, ﬁrst column).

Thus, expression of the Hgt4 sugar-binding domain in

S. cerevisiae confers a novel galactose response upon baker’s

yeast.

Galactose Induces MTH1 Expression in Diverse Fungi

C. albicans did not undergo a whole-genome duplication, so it

has one homolog of the

S. cerevisiae MTH1 and STD1 paralogs

(CaSTD1).

CaStd1

(orf19.6173) is 27% identical both to the

S. cerevisiae Std1 (43% similar) and to Mth1 (41% similar)

and harbors a conserved motif (SxSxxSSIFS, residues 62–71)

that is critical for glucose-induced phosphorylation of ScStd1

and ScMth1 (which leads to their degradation) [19].We

surmised that since Hgt4 functions as a galactose sensor in

C. albicans, CaSTD1 expression must not be induced by

galactose. Indeed, CaSTD1 expression is unaffected by galac-

tose (Table 1 and data not shown), a result conﬁrmed by

RT-PCR and RT-qPCR analyses (Figure S6 and Figure 4C,

respectively). To assess the evolutionary conservation of this

galactose response, we measured expression of MTH1 ortho-

logs in a diverse sampling of fungi spanning w200 million

years of evolution (Figure 4B). In all species tested except

C. albicans, expression of MTH1 is induced in response to

galactose (Figure 4C and Figure S6). Induction occurs even

in C. glabrata, which has lost the GAL4 gene, as well as in

K. lactis, which lacks canonical Gal4-binding sites in the

promoter of its MTH1 ortholog (Table S2). Galactose-induced

activation of ScMTH1 expression by ScGal4 in S. cerevisiae

[8] appears to antagonize the galactose signal generated by

Hgt4, and such antagonism is likely in the four fungi in the

S. cerevisiae to K. lactis clade that we analyzed.

These data illuminate the evolution of galactose sensing in

fungi. Sensing galactose through both the Gal4 and the

Hgt4-Snf3-Rgt2 type pathways seems imprudent, because it

would lead to crossrepression of genes in both pathways

(see Summary and Figure 1). Within the Ascomycetes,

Candida glabrata, Kluyveromyces waltii, and Ashbya gossypii

have no canonical galactose sensor because GAL4 and/or

GAL80 is absent, but they have also lost all galactose-utiliza-

tion-pathway enzymes (GAL1, GAL7, and GAL10) and thus

cannot utilize galactose in any case [10, 20, 21]. The Gal4-

mediated galactose-sensing pathway is intact in a few yeasts

that diverged before the duplication, such as K. lactis

and

kluyveri [22–24]. Debaromyces

hansenii and Pichia stipitus

have GAL4 homologs but no obvious GAL80 homologs. In

contrast, all of the Candida species that we surveyed (except

C. glabrata), as well as Yarrowia lipolytica and Lodderomyces

elongisporus, harbor genes encoding the enzymes for galac-

tose metabolism, but their GAL4 genes are more similar to

CaGAL4 (than to ScGAL4), and all lack a GAL80 functional

homolog. The implication is that the Ascomycetes that can

metabolize galactose but have no Gal4 or Gal80 regulators

utilize an Hgt4-like sensing pathway to control galactose-

response genes. This supports the notion that the Gal4-

Gal80 control circuit arose prior to the origin of the

Figure 1. Sugar-Sensing Pathways in C. albicans

and S. cerevisiae

Glucose signaling begins at the cell surface with

the sensors (CaHgt4, or ScSnf3 and ScRgt2)

and ends in the nucleus, with deactivation of

the Rgt1 transcriptional repressor [1, 11]. The

keystone proteins are the transcriptional core-

pressors (CaStd1, or ScStd1 and ScMth1), which

associate with both the sensor and the transcrip-

tional repressor, and it is the levels of these

proteins that translate the environmental signal

into gene-expression changes. Sugar binding to

a sensor activates yeast casein kinase (Yck),

which then phosphorylates Std1 and Mth1,

thereby marking them for ubiquitylation by the

SCF

Grr1

complex and dooming them to destruc-

tion by the proteasome. Depletion of the core-

pressors renders Rgt1 impotent, which results

in transcriptional derepression of downstream genes. In S. cerevisiae, galactose enters the cell, is phosphorylated, and binds (with ATP) to the Gal3 protein.

This complex binds and sequesters Gal80 and relieves the inhibition of the Gal4 transcriptional activator. In C. albicans, CaGal4 does not regulate the GAL

genes. Instead, galactose is sensed by the Hgt4 glucose sensor and probably also through Cph1 (a homolog of the S. cerevisiae Ste12).

Fungal Sugar Sensing

437

S. cerevisiae–K. lactis clade but after this clade and Candida,

species diverged from their common ancestor (Figure 4B,

white dot), and it suggests that Hgt4 represents an ancestral

sensor of galactose. In C. albicans, the altered speciﬁcity of

the Hgt4 glucose sensor, in combination with the absence of

a canonical Gal4 pathway, has enabled this fungus to sense

galactose through Hgt4.

C. albicans Std1 Functions in the Sugar-Sensing Pathway

Because the Gal4 signaling pathway is structured differently in

C. albicans, it was possible that the Hgt4 pathway had also

changed. If sugar sensing by C. albicans is analogous to

S. cerevisiae glucose sensing, the CaStd1 corepressor

should be a key protein in the pathway (Figure 1). Indeed it

is, because homozygous Dstd1 null mutant cells have the

Table 1. Genes Induced in Response to Sugars

Name ORF ID Galactose Fold Up Glucose Fold Up Annotation

GROUP I

HGT7 orf19.2023 32.2 30.8 Putative glucose transporter

QDR1 orf19.508 18.6 56.1 Antibiotic resistance transporter

AOX2 orf19.4773 13.7 8 Alternative oxidase

CRZ2 orf19.2356 10.1 12.8 Putative transcription factor

FET99 orf19.4212 10 19.1 Multicopper oxidase family

RHR2 orf19.5437 9.2 25.1 Putative glycerol 3-phosphatase

RNR22 orf19.1868 8.1 25.4 Ribonucleoside di-Phosphate reductase

TPO3 orf19.4737 7.4 16.4 Possible polyamine transporter

PDC11 orf19.2877 6.2 9.1 Similar to pyruvate decarboxylase

HGT6 orf19.2020 5.3 3.2 Putative glucose transporter

MNN22 orf19.3803 5.3 9.1 Golgi alpha-1, 2-mannosyltxferase

FMA1 orf19.6837 5.2 9.9 Membrane-assoc. protein

HAK1 orf19.6249 5.1 3.4 Putative potassium transporter

HXK2 orf19.542 5 8 Hexokinase II

TYE7 orf19.4941 4.4 5.7 Putative bHLH transcription factor

GDH3 orf19.4716 4.3 20.2 NADP-glutamate dehydrogenase

CMK1 orf19.5911 4.1 5 Ca2+/Calmodulin-dependent kinase

FET34 orf19.4215 3.8 6.8 Similar to multicopper ferroxidase

STP4 orf19.909 3.8 5.7 Putative transcription factor

AOX1 orf19.4774 3.7 3.3 Alternative oxidase

EHT1 orf19.3040 3.4 8.4 Similar to Eht1p

PFK1 orf19.3967 3.2 7 a-subunit of phosphofructokinase

CRP1 orf19.4784 3.2 6.9 Copper transporter

PFK2 orf19.6540 3.1 5.7 b-subunit of phosphofructokinase

PHO15 orf19.4444 3 5.8 4-nitrophenyl phosphatase

UBC15 orf19.5337 2.9 3 Ub-conjugation, DNA repair

MIG1 orf19.4318 2.9 3.2 Transcriptional repressor

AHP1 orf19.2762 2.9 6.3 Putative alkyl hydroperoxide reductase

ARG1 orf19.7469 2.7 4 Similar to argininosuccinate synthase

PHO113 orf19.2619 2.6 3.6 Constitutive acid phosphatase

NDE1 orf19.339 2.5 2.7 Putative NADH dehydrogenase

GPX2 orf19.85 2.5 2.25 Similar to glutathione peroxidase

ROD1 orf19.1509 2.4 3.8 Drug tolerance; Rgt1-repressed

FCR1 orf19.6817 2.3 2.8 Put. Zn-cluster transcription factor

OPT9 orf19.2584 2.1 3.1 Probable pseudogene

EBP7 orf19.5816 2 1.9 Stress-induced via Cap1p

ARG5 orf19.4788 2 5 Arginine biosynthetic enzyme

ARG4 orf19.6689 2 3.9 Argininosuccinate lyase

DOG1 orf19.3392 1.9 2.6 Put. 2-deoxygluc-6-phosphatase

YIM1 orf19.847 1.9 1.8 Similar to mitochondrial protease

GROUP II

HGT12 orf19.7094 6.74 0.07 Glucose, fructose, mannose transporter

HXT10 orf19.4384 2.6 0.21 Sugar transporter

HGT2 orf19.3668 2 0.02 Putative glucose transporter

GAL1 orf19.3670 6.1 0.8 Galactokinase

GAL10 orf19.3672 6 0.7 UDP-glucose 4-epimerase

GAL7 orf19.3675 5.9 0.9 UDP-hexose-1-P uridylyltransferase

GROUP III

PGA37 orf19.3923 3.2 1.1 Putative GPI-anchored protein

HSP30 orf19.4526 2.9 0.48 Similar To heat shock protein

STB3 orf19.203 2.3 1.4 Predicted Sin3 Binding protein

Fold induction indicates gene expression in each sugar relative to its expression in glycerol cultures. For all genes listed, p < 0.05 (Student’s t test). Unchar-

acterized open reading frames induced in galactose and glucose are shown in Table S4.

Genes induced in 2% galactose and in 2% glucose.

Genes induced in 2% galactose but repressed in 2% glucose.

Genes induced in 2% galactose, not induced in 2% glucose.

Current Biology Vol 19 No 5

438

same hyperﬁlamented morphology as Drgt1 mutant cells

(Figure S7A) and cells carrying the constitutively signaling

HGT4-1 mutation (Figure S7B), and this phenotype is reversed

by reintroduction of one wild-type allele into these cells

(Figure S7B). This result supports previous observations that

implicated the Hgt4 pathway in C. albicans ﬁlamentation [11,

12]. Furthermore, HGT7 expression is constitutive in the

Dstd1 mutant (just like in the HGT4-1 mutant), and reintroduc-

tion of one copy of CaSTD1 into this mutant reverses this

phenotype (Figure 5A). Thus, although the galactose-sensing

pathways are completely different between C. albicans and

S. cerevisiae, the glucose-sensing pathway remains the

same (Hgt4-CaStd1-CaRgt1).

Examining CaStd1 function in C. albicans sheds light on the

separate functions of the S. cerevisiae paralogs. CaSTD1

expression resembles that of ScSTD1, not ScMTH1:itis

induced by glucose but not by galactose (Figure 5B and

Figure 4C, respectively) [25]. This implies that ScStd1 has

a more ancestral role, and ScMth1 a more derived role, in

this signal-transduction pathway. The Hgt4-Snf3-Rgt2 sugar-

sensing pathway may be universally involved in fungal

morphology; disrupting ScMTH1 represses ﬁlamentous

growth in baker’s yeast in the S1278b pseudohyphal strain

[26]. Additional studies on pre- and postduplication yeast

species will be necessary for determining whether Mth1 and

Std1 function redundantly, cooperatively, or in opposition to

each other and whether they affect fungal morphogenesis

throughout this kingdom.

It seems clear that the glucose- and galactose-sensing

systems in fungi work together as a network to regulate tran-

scription of genes such as GAL1 in C. albicans and HXT1 in

S. cerevisiae. In fact, transcriptional regulation of the HXT

genes in S. cerevisiae is the result of at least seven intercon-

nected signal-transduction cascades: (1) glucose sensing

through Snf3 or Rgt2 [27], (2) sugar sensing through the Gpr1

G protein-coupled receptor [28], (3) osmosensing through

the Hog1 MAP kinase pathway [29], (4) glucose repression

mediated by Mig1 and Mig2 [6],

(5)

the TOR1 protein kinase

pathway [30], (6) oxygen availability [31], and, ﬁnally, (7)

galactose sensing through Gal4. These signal-transduction

Figure 2. Hgt4 Regulates Galactose-Induced Genes

Log-phase cultures of wild-type (BWP17), Dhgt4 (CM9 and CM10), or

Dhgt12 (CM64) cells were split and incubated in fresh media containing

5% galactose or 5% glycerol at 30



C for 2 hr. Total RNA was reverse tran-

scribed and PCR ampliﬁed with primers for HGT7 (orf19.2023), QDR1

(orf19.508), AOX2 (orf19.4773), CMK1 (orf19.5911), HXK2 (orf19.542), and

ACT1 (orf19.5007). Control reactions lacking reverse transcriptase yielded

no products (not shown).

Figure 3. HGT7 Is Induced in Response to Galactose

(A) The HGT7 promoter was fused to Streptococcus thermophilus lacZ

gene, and this construct was integrated into the C. albicans genome at

the native HGT7 locus. Cells with HGT7::HGT7-lacZ (CM79 and CM80)

were grown in glycerol media, split, and incubated at 30



C for 2 hr in fresh

media containing glycerol or 0.04% (top panel) or 1.6% (bottom panel) of

the sugars indicated. Cells were lysed and assayed for b-galactosidase

activity (in quadruplicate [top] or in triplicate [bottom]), and the results

were normalized to the lacZ activity in the glycerol media. Data are pre-

sented as mean 6 1 SD.

(B) Cells with HIS1::HGT7-lacZ (HGT4 strains CM230 and CM231 and Dhgt4

strains CM232 and CM233) were grown in media with glycerol as carbon

source, split, and incubated at 30



C for 2 hr in fresh media lacking histidine

but containing glycerol or the sugar indicated (n = 10 for HGT4; n = 10 for

Dhgt4). Black bar: 0.04% glucose; gray bar: 1.6% glucose; striped bar:

0.04% galactose; white bar: 1.6% galactose. All values were normalized

to activity in glycerol and expressed as the percentage of the maximum

response in 0.04% glucose. Data are presented as mean 6 1 SD.

Fungal Sugar Sensing

439

pathways provide a malleable framework for response to

extracellular nutrients.

Supplemental Data

Supplemental Data include Supplemental Experimental Procedures, seven

ﬁgures, and four tables and can be found with this article online at http://

www.current-biology.com/supplemental/S0960-9822(09)00628-9.

Acknowledgments

We thank Chris Todd Hittinger and Chandra Tucker for critical reading of this

manuscript and Jim Dover and Christine Carle for technical assistance. This

work was supported by a grant from the National Institute of Diabetes and

Digestive and Kidney Diseases (K01DK077878 to V.B.), by grants from the

National Institute of General Medical Sciences (F32GM076967 to J.S. and

R01GM32540 to M.J.), and by funds provided to M.J. by the James

S. McDonnell Foundation.

Received: October 1, 2008

Revised: January 12, 2009

Accepted: January 15, 2009

Published online: February 26, 2009

Figure 4. C. albicans HGT4 Confers a Novel Galactose Response

upon S. cerevisiae

(A) S. cerevisiae strains were grown in media containing glycerol, cell densi-

ties were normalized, and the culture was split and incubated overnight at



C in fresh media containing 5% glycerol or 5% galactose, then lysed,

and b-galactosidase activity was assayed (see Supplemental Experimental

Procedures). Data are the average of biological duplicates. White bars:

wild-type cells + pRS316 vector (YM7642); black bars: wild-type cells

+Hgt4-chimera (YM7643); gray bars: Dgal4 cells + pRS316 vector (YM7644);

blue bars: Dgal4 cells + Hgt4-chimera (YM7645). Data are presented as

mean 6 1 SD.

(B) MTH1 orthologs are galactose-induced in diverse fungi. A phylogenetic

tree showing the relationship of yeasts spanning w200 million years of

evolution is shown [32–35]. Characteristics of the galactose-sensing path-

ways in these species are described in Table S2. The black circle represents

a whole-genome duplication event, the white circle represents the

proposed appearance of the Gal4-Gal80 gene-regulatory mechanism, and

asterisks indicate the species analyzed in (C).

fresh media containing 5% glycerol or 5% galactose at 30



C for 3 hr.

RT-PCR was performed on total RNA with the use of species-speciﬁc

primers for either ACT1 or the MTH1/STD1 ortholog (fungal strains are

described in Supplemental Experimental Procedures). First-strand cDNAs

served as templates for quantitative PCR. Each MTH1/STD1 signal was

normalized to the ACT1 signal in that sample, and the DDCt values are

expressed as ‘‘Fold Induction’’ of expression in galactose relative to expres-

sion in glycerol (2

DDCt

). Data are presented as mean 6 1 SD. Separate exper-

iments were performed with the use of semiquantitative PCR for conﬁrma-

tion of the results (see Figure S6).

Figure 5. CaSTD1 and ScSTD1 Function Similarly

(A) CaSTD1 plays a role in the HGT4 pathway. Isogenic strains [HGT4 (CM87)

compared to HGT4-1 (CM36) and Dstd1 (CM222) compared to STD1

(CM224)] were grown at 30



C to log phase in media containing glycerol.

Cells were harvested and snap frozen, and total RNA was puriﬁed for

RT-PCR analysis of HGT7 (orf19.2023) or ACT1 (orf19.5007).

(B) CaSTD1 is glucose-induced. C. albicans cells (SC5314) were grown to

log phase in media containing glycerol, then incubated at 30



Cfor2hrin

fresh media with glycerol (gly) or with the indicated concentrations of

glucose (0 indicates no carbon source). Cells were harvested and snap

frozen, and total RNA was puriﬁed for RT-PCR analysis with the use of

primers for CaSTD1 (orf19.6173) or ACT1 (orf19.5007).

Current Biology Vol 19 No 5

440

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