Yeast’s ADH2 Gene Reveals a Regulatory Network That Could Boost Bioethanol Production

A new review published in Current Genetics by Pratima Sarkar, Rohan Nath, Prity Adhikary and Arindam Bhattacharjee now assembles the scattered evidence into a single regulatory map centred on one gene: ADH2. The gene encodes Alcohol dehydrogenase II, or Adh2p, an enzyme the authors describe as the only one in yeast that initiates the conversion of ethanol, the end product of fermentation, back into acetaldehyde.

The review’s central argument is that ADH2 is not governed by a lone switch but by a layered network of glucose-sensing pathways. Each responds differently to glucose, the yeast’s preferred energy source, and each ultimately reaches down to the promoter region of ADH2, where a pair of upstream activating sequences called UAS1 and UAS2 await their protein visitors. When glucose runs low, the gates open and ADH2 expression rises, allowing the cell to metabolize ethanol and other alternative carbon sources such as glycerol.

The ADH2 promoter carries UAS1, a 22-base-pair palindromic sequence, and UAS2, both located upstream of the structural gene. The two sequences work synergistically to regulate transcription. Under low or absent glucose, the transactivator ADR1 binds to UAS1 while CAT8 binds to UAS2, enabling expression. Under high glucose, these major regulators are unable to bind the promoter, restricting ADH2 transcription. The review identifies ADR1, CAT8 and CCR4 as positive regulators of the gene, with CRE1 and CRE2 acting as negative regulators, while SNF1 and TOR operate as global transcription factors above them.

The Snf1 pathway is the fungal counterpart of the mammalian AMP-activated kinase, a serine-threonine kinase that coordinates cellular energy balance and the response to nutritional stress. In S. cerevisiae it is required for adaptation to glucose limitation and for growth on non-fermentable carbon sources. The Snf1 complex is heterotrimeric, containing a catalytic alpha subunit, a regulatory beta subunit drawn from Sip1p, Sip2p or Gal83p, and the gamma subunit Snf4p. The complex is constitutively expressed regardless of glucose concentration and remains cytoplasmic when glucose is high. Under limiting glucose, upstream kinases Sak1p, Elm1p or Tos3p phosphorylate the complex at threonine 210, an event essential for activation and for its effects on downstream pathways. Strains lacking all three upstream kinases, or lacking Snf1 itself, show major growth defects when grown on limiting glucose or on ethanol and glycerol.

There, the protein phosphatase Glc7p, working with the adaptor Reg1p, dephosphorylates Snf1p; loss of Reg1p can leave Snf1p constitutively active even under repressing conditions, producing constitutive ADH2 expression. When activated by phosphorylation, Snf1p enters the nucleus and modifies Mig1p, a repressor that in its unphosphorylated state blocks the promoter regions of ADR1 and CAT8. Phosphorylation exports Mig1p from the nucleus, freeing ADR1 and CAT8 to be transcribed and to act downstream on ADH2. ADR1 itself is activated by phosphorylation at its serine 230 region, indirectly by Snf1p, which promotes its binding to the ADH2 promoter at a site 230 base pairs from the transcription start. Strains carrying mutant ADR1 cannot fully derepress ADH2 even without glucose, yet residual transcription hints at other regulators. Phosphorylation of Adr1p can also be carried out by Sch9p, a kinase mainly regulated by TOR, an early sign of crosstalk between the two great nutrient pathways.

Tor proteins form two complexes, TORC1 and TORC2. TORC1 responds to amino acids and glucose, stimulating ribosome biogenesis and protein synthesis while blocking autophagy through 4E-binding proteins and activating the S6 kinase counterpart Sch9p. TORC1 is assembled from Kog1p, Tco89p and Lst8p; TORC2, concerned with lipid metabolism and the actin cytoskeleton, contains Lst8p, Avo1p, Avo2p, Avo3p, Bit61p and Bit2p. TORC1 activity falls during glucose depletion, triggering autophagy and resource conservation. Among the transcription factors through which TORC1 indirectly influences ADH2 are Hap4p, Sfp1p and Gat1p, but the key role belongs to Sch9p, the most important target of TORC1. In a striking experimental result cited from earlier work, strains with deleted SCH9 showed a tenfold decrease in Adh2p activity, marking Sch9p as a positive regulator of the gene. Sch9p also influences CAT8 and HAP4, tying ADH2 regulation indirectly to TORC1 through multiple routes, though the precise role of Sch9p phosphorylation under high and low glucose in repressing or derepressing ADH2 still needs further analysis.

Downstream of TOR, the kinase Rim15p becomes active when the pathway is blocked in low-glucose situations, activating the transcription factors Msn2/4p and Gis1p and raising the production of stress-tolerance and stationary-phase genes. The review suggests this may indirectly favour ADH2 induction by creating a cellular environment conducive to it, even though Rim15p’s main effects are on stress resistance rather than on the gene itself. HAP4, which activates genes for mitochondrial biogenesis and oxidative phosphorylation, acts downstream of both PKA and TOR; its overexpression raised respiration capacity of growing cells by 10 percent, while its deletion left growth rate unchanged but cut glucose uptake by 75 percent, indicating reduced respiration. Deletion of SCH9 raises HAP4 mRNA levels and respiratory metabolism even in high glucose, whereas Sch9p overexpression suppresses both. The calcium-responsive factor Crz1p, upregulated under heat and ethanol stress, and the GATA factor Gln3p, whose nuclear entry is governed independently by TORC1 and Snf1p, round out the network, although deletion of GLN3 reportedly had no effect on ADH2 expression even on alternative carbon sources.

CCR4, also called CCR4-NOT, is one of the largest protein groups involved in transcription and includes Ccr4p, Caf1p, five NOT proteins and two others. Spt10p and Spt6p, allelic to Cre1p and Cre2p, act as negative regulators of CCR4. When CRE1 and CRE2 mutant strains were grown in glucose-containing media, ADH2 was found to be expressed, indicating that these genes normally repress it; on their deletion, ADH2 was released from glucose repression, allowing CCR4 protein to bind the promoter and drive transcription. Notably, this regulation appears independent of the upstream activating sequences and of Adr1p binding, although reduced ADH2 expression was observed when UAS2 was multimerized or its orientation changed, implying that CCR4 binds UAS2 to control the gene independently of Adr1p.

Disrupting the ADH2 gene has produced a 1.74-fold increase in ethanol productivity in published studies, and Fast TALEN technology applied to the gene yielded roughly 52 to 55 percent higher ethanol yield. Deleting Mig1p, the repressor acting downstream of Snf1, produced 4.06 plus or minus 0.14 grams per litre of ethanol and improved acetic acid tolerance. Deleting HAP4 or RIM15 also raised ethanol productivity, with RIM15 deletion mutants retaining cell viability in high glucose. A CRISPR-Cas9 deletion of FPS1, ADH2 and GPD2 produced ethanol 0.18 percent above wild type, and overexpression of the stress factor Znf1p lifted productivity by 14 to 24 percent while improving osmotic stress adaptation. The gene’s promoter has even been harnessed as a tool: driving the first stage of lutein biosynthesis under the ADH2 promoter delivered 19.92 milligrams per litre, the highest concentration reported from a heterologous production system, and the promoter has guided lycopene biosynthesis genes in cells grown under low glucose.

ADH2 expression is shaped not only by transcription factors but by post-translational modification and chromatin remodelling; histone acetylation and deacetylation at the ADH2 promoter facilitate Adr1p binding even under repressing conditions, though that step alone is insufficient for derepression, and Snf1 plays a direct role in chromatin remodelling while CAT8 and ADR1 stabilize transcription initiation complexes. Post-translational roles for NUP159, LSM6 and EBS1, all linked to CCR4, also affect ADH2 expression. Comparative biology adds perspective: in Pichia stipitis, ADH2 is regulated by oxygen rather than glucose, and Kluyveromyces lactis carries four ADH genes each capable of ethanol utilization. The authors argue that further work on epigenetic factors and glucose effects is needed before the network can be exploited fully at industrial scale.

Subject of Research: Regulatory networks controlling ADH2 expression in Saccharomyces cerevisiae

Article Title: Possible regulatory network and associated pathways governing the expression of ADH2 in Saccharomyces cerevisiae

Article References: Sarkar, P., Nath, R., Adhikary, P., & Bhattacharjee, A. (2025). Possible regulatory network and associated pathways governing the expression of ADH2 in Saccharomyces cerevisiae. Current Genetics, 71(1), Article 15. https://doi.org/10.1007/s00294-025-01321-0

Image Credits: AI Generated

DOI: 10.1007/s00294-025-01321-0

Keywords: Saccharomyces cerevisiae, ADH2, bioethanol, glucose repression, Snf1 kinase, TOR pathway, CCR4 complex, transcription factors, yeast metabolism, ethanol utilization, genetic engineering, fermentation