The G(1)/S transition is a critical control point for cell proliferation and 
involves essential transcription complexes termed SBF and MBF in Saccharomyces 
cerevisiae or MBF in Schizosaccharomyces pombe. In the fungal pathogen Candida 
albicans, G(1)/S regulation is not clear. To gain more insight into the G(1)/S 
circuitry, we characterized Swi6p, Swi4p and Mbp1p, the closest orthologues of 
SBF (Swi6p and Swi4p) and MBF (Swi6p and Mbp1p) components in S. cerevisiae. The 
mbp1Δ/Δ cells showed minor growth defects, whereas swi4Δ/Δ and swi6Δ/Δ yeast 
cells dramatically increased in size, suggesting a G(1) phase delay. Gene set 
enrichment analysis (GSEA) of transcription profiles revealed that genes 
associated with G(1)/S phase were significantly enriched in cells lacking Swi4p 
and Swi6p. These expression patterns suggested that Swi4p and Swi6p have 
repressing as well as activating activity. Intriguingly, swi4Δ/Δ swi6Δ/Δ and 
swi4Δ/Δ mbp1Δ/Δ strains were viable, in contrast to the situation in S. 
cerevisiae, and showed pleiotropic phenotypes that included multibudded yeast, 
pseudohyphae, and intriguingly, true hyphae. Consistently, GSEA identified 
strong enrichment of genes that are normally modulated during C. albicans-host 
cell interactions. Since Swi4p and Swi6p influence G(1) phase progression and 
SBF binding sites are lacking in the C. albicans genome, these factors may 
contribute to MBF activity. Overall, the data suggest that the putative G(1)/S 
regulatory machinery of C. albicans contains novel features and underscore the 
existence of a relationship between G(1) phase and morphogenetic switching, 
including hyphal development, in the pathogen.