The C-terminal region of the Hot1 transcription factor binds GGGACAAA-related sequences in the promoter of its target genes

Maltose accumulation-induced cell death in Saccharomyces cerevisiae

Pretreatment of lignocellulose yields a complex sugar mixture that potentially can be converted into bioethanol and other chemicals by engineered yeast. One approach to overcome competition between sugars for uptake and metabolism is the use of a consortium of specialist strains capable of efficient conversion of single sugars. Here, we show that maltose inhibits cell growth of a xylose-fermenting specialist strain IMX730.1 that is unable to utilize glucose because of the deletion of all hexokinase genes. The growth inhibition cannot be attributed to a competition between maltose and xylose for uptake. The inhibition is enhanced in a strain lacking maltase enzymes (dMalX2) and completely eliminated when all maltose transporters are deleted. High-level accumulation of maltose in the dMalX2 strain is accompanied by a hypotonic-like transcriptional response, while cells are rescued from maltose-induced cell death by the inclusion of an extracellular osmolyte such as sorbitol. These data suggest that maltose-induced cell death is due to high levels of maltose uptake causing hypotonic-like stress conditions and can be prevented through engineering of the maltose transporters. Transporter engineering should be included in the development of stable microbial consortia for the efficient conversion of lignocellulosic feedstocks.

Glycerol uptake and synthesis systems contribute to the osmotic tolerance of Kluyveromyces marxianus

The accumulation of glycerol is essential for yeast viability upon hyperosmotic stress. In this study, the STL1 homolog KmSTL1, encoding a putative glycerol transporter contributing to cell osmo-tolerance, was identified in Kluyveromyces marxianus NBRC1777. We constructed the KmSTL1, KmGPD1, and KmFPS1 single-deletion mutants and the KmSTL1/KmGPD1 and KmSTL1/KmFPS1 double-deletion mutants of K. marxianus. Deletion of KmSTL1 or KmGPD1 resulted in K. marxianus cell sensitization to hyperosmotic stress, whereas deletion of KmFPS1 improved stress tolerance. The expression of KmSTL1 was osmotically induced, whereas that of KmFPS1 was osmotically inhibited. The expression of KmGPD1 was constitutive and continuous in the ΔKmSTL1 mutant strain but inhibited in the ΔKmFPS1 mutant strain due to feedback suppression by glycerol. In summary, our findings indicated that K. marxianus would increase glycerol synthesis by increasing GPD1 expression, increase glycerol import from the extracellular environment by increasing STL1 expression, and reduce glycerol efflux by reducing FPS1 expression under hyperosmotic stress.

Functional and expression studies of two novel STL1 genes of the osmotolerant and glycerol utilization yeast Candida glycerinogenes

Candida glycerinogenes is an osmotolerant yeast used for commercial glycerol production, as well as a glycerol utilization yeast which produces high biomass on glycerol medium. In the present study, two STL1 homologues CgSTL1 and CgSTL2 encoding the putative glycerol transporters were identified, and their products were found to be localized to plasma membranes by tagging GFP protein. The functions of CgSTL1 and CgSTL2 on glycerol transport were confirmed by their expression in S. cerevisiae STL1 null mutant and simultaneous deletion in C. glycerinogenes. The expression of CgSTL1 were osmotic-induced, whereas that of CgSTL2 was constitutive. Over-expression of CgSTL1 and CgSTL2 in C. glycerinogenes resulted in improved glycerol consumption rate and cell growth. Our study provided more details on the glycerol transporter of C. glycerinogenes, the potential cell factory for using glycerol as a carbon source.

A farnesoic acid-responsive transcription factor, Hot1, regulates yeast-hypha morphogenesis in Candida albicans

Candida albicans hyphal formation is inhibited by a quorum-sensing molecule, farnesoic acid, which accumulates in the medium as the cells proliferate. We recently showed that Pho81 is essential for the inhibition of hyphal growth by farnesoic acid. Here, we describe a newly identified regulator, Hot1, which increases the expression of PHO81. The binding site of Hot1 in the PHO81 promoter region was identified by DNase I protection assay. The hot1Δ mutant grows extensively as filaments. Furthermore, the inhibition of hyphal formation and the repression of major signaling pathway components in response to farnesoic acid are defective in hot1Δ mutant cells. These data suggest an important role for HOT1 in the inhibition of hyphal development by farnesoic acid in this fungus.

Hot1 factor recruits co-activator Sub1 and elongation complex Spt4/5 to osmostress genes

Hyperosmotic stress response involves the adaptative mechanisms needed for cell survival. Under high osmolarity conditions, many stress response genes are activated by several unrelated transcription factors that are controlled by the Hog1 kinase. Osmostress transcription factor Hot1 regulates the expression of several genes involved in glycerol biosynthesis, and the presence of this transcription factor in their promoters is essential for RNApol II recruitment. The physical association between Hog1 and Hot1 activates this transcription factor and directs the RNA polymerase II localization at these promoters. We, herein, demonstrate that physical and genetic interactions exist between Hot1 and several proteins involved in transcriptional and posttranscriptional processes: for example, transcription co-activator Sub1 and elongation complex Spt4/5. The results presented in this work demonstrate that Hot1 enrichment is not detected through the coding regions of its target genes and rule out a direct role in transcription elongation. Instead, other data presented herein indicate a key function of the Hot1 transcription factor in the recruitment of these proteins to the promoter or the 5′-coding region of the genes under its control.