Multiple Mechanisms Mediating the Starvation Induced Activation of Recombination at HIS4 in Saccharomyces Cerevisiae

Meiotic recombination occurs at relatively high levels at specific regions in the genome called hotspots. The transcription factor-dependent hotspots (α-hotspots) have been widely studied in yeast, and are beginning to be elucidated in mammals. The HIS4 hotspot activity in Saccharomyces cerevisiae requires binding of Bas1p, Bas2p, Rap1p and Gcn4p. Bas1p acts in conjunction with Bas2p to regulate basal level of transcription of their target genes, and can be stimulated under conditions of adenine starvation and accumulation of metabolites AICAR and SAICAR from the purine biosynthesis pathway. Gcn4p activates transcription of yeast genes in response to starvation for amino acids and purines. This study focused on the influence of nutritional starvation on HIS4 hotspot activity, and different mechanisms mediating this effect. Our data suggests that deletion of genes known to accumulate AICAR/SAICAR can stimulate recombination at HIS4 in a Bas1p-dependent manner. Furthermore, intracellular and extracellular starvation for adenine and amino acids also activates recombination at HIS4. In addition, moderate levels of starvation only affect recombination when chromatin is already hyperacetylated, by the inactivation of the Set2p methyltransferase. Bas1p plays an essential role in mediating the effect of starvation and the set2 mutation on recombination. We showed that Gcn4p is not required for HIS4 hotspot activity, but plays a modest role in the effect of starvation in an adenine auxotrophic strain. Additionally, the starvation effect is also mediated by an as yet unknown factor independent from Bas1p/Bas2p and Gcn4p. This work provides additional information regarding the regulation of a transcription factor-dependent hotspot activity, and factors influencing its activation. Furthermore, data in this study indicate that BAS1, and not BAS2 exhibit haploinsufficiency with respect to its function in activating meiotic recombination. This implies that Bas1p is rate-limiting for HIS4 hotspot activity.