Synergy of Hir1, Ssn6, and Snf2 global regulators is the functional determinant of a Mac1 transcriptional switch in S. cerevisiae copper homeostasis

The essential liaison of two copper proteins: the Cu-sensing transcription factor Mac1 and the Cu/Zn superoxide dismutase Sod1 in Saccharomyces cerevisiae

Although copper is an essential trace element for cell function and viability, its excess can lead to protein oxidation, DNA cleavage, and ultimate cell damage. Cells have established a variety of regulatory mechanisms to ensure copper ion homeostasis. In Saccharomyces cerevisiae, copper sensing and response to copper deficiency are regulated by the transcription factor Mac1. Our group has previously reported that in addition to copper, several chromatin proteins modulate Mac1 functionality. In this study, based on a synthetic growth deficiency phenotype, we showed that the Cu/Zn superoxide dismutase Sod1 plays an important role in Mac1 transcriptional activity, in unchallenged nutrient-rich growth conditions. Sod1 is a multipotent cytoplasmic and mitochondrial enzyme, whose main known function is to detoxify the cell from superoxide ions. It has been previously reported that Sod1 also enters the nucleus and affects the transcription of several genes, some of which are involved in copper homeostasis under Cu-depleted (Wood and Thiele in J Biol Chem 284:404-413, 2009) or only under specific oxidative stress conditions (Dong et al. Mol Cell Biol 33:4041-4050, 2013; Tsang et al. Nar Commun 8:3446, 2014). We have shown that Sod1 physically interacts with Mac1 transcription factor and is important for the transactivation as well as its DNA-binding activities. On the other hand, a constitutively active mutant of Mac1 is not affected functionally by the Sod1 ablation, pointing out that Sod1 contributes to the maintenance of the copper-unchelated state of Mac1. In conclusion, we showed that Sod1-Mac1 interaction is vital for Mac1 functionality, regardless of copper medium deficiency, in unchallenged growth conditions, and we suggest that Sod1 enzymatic activity may modify the redox state of the cysteine-rich motifs in the Mac1 DNA-binding and transactivation domains.

Yeast chromatin remodeling complexes and their roles in transcription

The nucleosome is a small unit of chromatin, which is dynamic in eukaryotes. Chromatin conformation and post-translational modifications affect nucleosome dynamics under certain conditions, playing an important role in the epigenetic regulation of transcription, replication and reprogramming. The Snf2 remodeling family is one of the crucial remodeling complexes that tightly regulate chromatin structure and affect nucleosome dynamics. This family alters nucleosome positioning, exchanges histone variants, and assembles and disassembles nucleosomes at certain locations. Moreover, the Snf2 family, in conjunction with other co-factors, regulates gene expression in Saccharomyces cerevisiae. Here we first review recent findings on the Snf2 family remodeling complexes and then use some examples to illustrate the cooperation between different members of Snf2 family, and the cooperation between Snf2 family and other co-factors in gene regulation especially during transcription initiation.

Distinct associations of the Saccharomyces cerevisiae Rad9 protein link Mac1-regulated transcription to DNA repair

While it is known that ScRad9 DNA damage checkpoint protein is recruited to damaged DNA by recognizing specific histone modifications, here we report a different way of Rad9 recruitment on chromatin under non DNA damaging conditions. We found Rad9 to bind directly with the copper-modulated transcriptional activator Mac1, suppressing both its DNA binding and transactivation functions. Rad9 was recruited to active Mac1-target promoters (CTR1, FRE1) and along CTR1 coding region following the association pattern of RNA polymerase (Pol) II. Hir1 histone chaperone also interacted directly with Rad9 and was partly required for its localization throughout CTR1 gene. Moreover, Mac1-dependent transcriptional initiation was necessary and sufficient for Rad9 recruitment to the heterologous ACT1 coding region. In addition to Rad9, Rad53 kinase also localized to CTR1 coding region in a Rad9-dependent manner. Our data provide an example of a yeast DNA-binding transcriptional activator that interacts directly with a DNA damage checkpoint protein in vivo and is functionally restrained by this protein, suggesting a new role for Rad9 in connecting factors of the transcription machinery with the DNA repair pathway under unchallenged conditions.