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The DAN/TIR genes encode nine cell wall mannoproteins in Saccharomyces cerevisiae which are expressed during anaerobiosis (DAN1, DAN2, DAN3, DAN4, TIR1, TIR2, TIR3, TIR4, and TIP1). Most are expressed within an hour of an anaerobic shift, but DAN2 and DAN3 are expressed after about 3 h. At the same time, CWP1 and CWP2, the genes encoding the major mannoproteins, are down-regulated, suggesting that there is a programmed remodeling of the cell wall in which Cwp1 and Cwp2 are replaced by nine anaerobic counterparts. TIP1, TIR1, TIR2, and TIR4 are also induced during cold shock. Correspondingly, CWP1 is downregulated during cold shock. As reported elsewhere, Mox4 is a heme-inhibited activator, and Mot3 is a heme-induced repressor of the DAN/TIR genes (but not of TIP1). We show that CWP2 (but not CWP1) is controlled by the same factors, but in reverse fashion-primarily by Mot3 (which can function as either an activator or repressor) but also by Mox4, accounting for the reciprocal regulation of the two groups of genes. Disruptions of TIR1, TIR3, or TIR4 prevent anaerobic growth, indicating that each protein is essential for anaerobic adaptation. The Dan/Tir and Cwp proteins are homologous, with the greatest similarities shown within three subgroups: the Dan proteins, the Tip and Tir proteins, and, more distantly, the Cwp proteins. The clustering of homology corresponds to differences in expression: the Tip and Tir proteins are expressed during hypoxia and cold shock, the Dan proteins are more stringently repressed by oxygen and insensitive to cold shock, and the Cwp proteins are oppositely regulated by oxygen and temperature.The cell wall of Saccharomyces cerevisiae is a rigid structure which determines cell morphology and also serves as a protective barrier, providing mechanical protection and enabling selective uptake of macromolecules (1a, 4, 7). A major component of the cell wall is mannoprotein, comprising about 40% of its mass. Mannoproteins are believed to be determinants of cell wall permeability (31), and certain ones are also essential for developmental events such as mating and transition to hyphal growth (18,24). Some mannoproteins can be extracted from the cell wall with detergent; others are covalently bound but can be released with glucanase (7). Proteins in the latter category have common features, including a region rich in serine and threonine, a glycosylphosphatidylinositol (GPI) anchor attachment signal at C termini (3,8), and an endoplasmic reticulum localization sequence; several also have a PAU domain. This segment of about 100 amino acids is shared among a group of proteins known as "seripauperins" (29).The Cwp2 mannoprotein (28) is one of the most abundant proteins of the cell wall and is believed to play a role in its stabilization, along with another homologous constituent, Cwp1 (23, 27, 28). While Cwp1 and Cwp2 are expressed under normal growth conditions, other mannoproteins are expressed in response to environmental stress. The most extensive response is the induction of several homologou...

The ROXI gene encodes a product implicated in the regulation of heme-repressed and heme-induced genes in Saccharomyces cerevisiae. The gene has been cloned and shown to code for a 1.4-kilobase transcript. The cloned gene was used to construct a null mutant to determine the role of ROXI in regulating the expression of several heme-regulated genes. Constitutive expression of ANBI (a heme-repressed gene) was observed in the null strain, indicating that ROXI codes for a repressor or a facilitator of repression. Enhancement of expression of CYC7 in the null strain indicated that the ROXI factor is required for repression of CYC7 to its normal low level of expression, consistent with evidence that CYC7 has a hybrid heme-induced, heme-repressed regulatory mechanism. The null mutation had only a slight negative effect on expression of the heme-induced genes CYCI and tr-i (a heme-induced homolog of ANBI), suggesting that the ROXI factor is not directly involved in their regulation despite the existence of an unusual roxi mutation (roxi-al) causing constitutive expression of this group. The respiratory competence of the null mutant indicates that ROXI is not a respiratory factor. ROXI expression was found to be induced by heme, indicating that the heme repression of ANBI and its family is the result of a cascade in which heme induces a repression factor which keeps the family of heme-repressed genes inactive during aerobic growth. The roxi-al allele had earlier been shown to cause constitutive expression of the family of heme-induced respiratory genes. This allele was found to cause constitutive expression of the ROXI transcript itself, indicating that ROXI is in the major heme-induced regulon.