The common fungal pathogen, Candida albicans, can grow either as single cel
ls or as filaments (hyphae), depending on environmental conditions. Several
transcriptional regulators have been identified as having key roles in con
trolling filamentous growth, including the products of the TUP1, CPH1, and
EFG1 genes. We show, through a set of single, double, and triple mutants, t
hat these genes act in an additive fashion to Control filamentous growth, s
uggesting that each gene represents a separate pathway of control. We also
show that environmentally induced filamentous growth can occur even in the
absence of all three of these genes, providing evidence for a fourth regula
tory pathway. Expression of a collection of structural genes associated wit
h filamentous growth, including HYR1, ECE1, HWP1, ALS1, and CHS2, was monit
ored in strains lacking each combination of TUP1, EFG1, and CPH1. Different
patterns of expression were observed among these target genes, supporting
the hypothesis that these three regulatory proteins engage in a network of
individual connections to downstream genes and arguing against a model wher
eby the target genes are regulated through a central filamentous growth pat
hway. The results suggest the existence of several distinct types of filame
ntous forms of C. albicans, each dependent on a particular set of environme
ntal conditions and each expressing a unique set of surface proteins.