Regulatory mechanisms controlling expression of the DAN/TIR mannoprotein genes during anaerobic remodeling of the cell wall in Saccharomyces cerevisiae

The DAN/TIR genes of Saccharomyces cerevisiae encode homologous mannoprotei ns, some of which are essential for anaerobic growth. Expression of these g enes is induced during anaerobiosis and in some cases during cold shock. We show that several heme-responsive mechanisms combine to regulate DAN/TIR g ene expression. The first mechanism employs two repression factors, Mox1 an d Mox2, and an activation factor, Mox4 (for mannoprotein regulation by oxyg en). The genes encoding these proteins were identified by selecting for rec essive mutants with altered regulation of a dan1::ura3fusion. MOX4 is ident ical to UPC2, encoding a binucleate zinc cluster protein controlling expres sion of an anaerobic sterol transport system. Mox4/Upc2 is required for exp ression of all the DAN/TIR genes. It appears to act through a consensus seq uence termed the AR1 site, as does Mox2. The noninducible mox4 Delta allele was epistatic to the constitutive mox1 and mox2 mutations, suggesting that Mox1 and Mox2 modulate activation by Mox4 in a heme-dependent fashion. Mut ations in a putative repression domain in Mox4 caused constitutive expressi on of the DAN/TIR genes, indicating a role for this domain in heme repressi on. MOX4 expression is induced both in anaerobic and cold-shocked cells, so heme may also regulate DAN/TIR expression through inhibition of expression of MOX4. Indeed, ectopic expression of MOX4 in aerobic cells resulted in p artially constitutive expression of DAN1. Heme also regulates expression of some of the DAN/TIR genes through the Rox7 repressor, which also controls expression of the hypoxic gene ANB1. In addition Rox1, another heme-respons ive repressor, and the global repressors Tup1 and Ssn6 are also required fo r full aerobic repression of these genes.