Spore germination is a process whereby spores exit dormancy to become competent for mitotic cell division. In Schizosaccharomyces pombe, one critical step of germination is the formation of a germ tube that hatches out the spore wall in a stage called outgrowth. Here, we show that iron deficiency blocks the outgrowth of germinating spores. The siderophore synthetase Sib1 and the ornithine N5-oxygenase Sib2 participate in ferrichrome biosynthesis, whereas Str1 functions as a ferrichrome transporter. Expression profiles of sib1+, sib2+, and str1+ transcripts reveal that they are induced shortly after induction of germination and their expression remains upregulated throughout the germination program under low-iron conditions. sib1Δ sib2Δ mutant spores are unable to form a germ tube under iron-poor conditions. Supplementation with exogenous ferrichrome suppresses this phenotype when str1+ is present. Str1 localizes at the contour of swollen spores 4 hr after induction of germination. At the onset of outgrowth, localization of Str1 changes and it moves away from the mother spore to primarily localize at the periphery of the new daughter cell. Two conserved Tyr residues (Tyr553 and Tyr567) are predicted to be located in the last extracellular loop region of Str1. Results show that these amino acid residues are critical to ensure timely completion of the outgrowth phase of spores in response to exogenous ferrichrome. Taken together, the results reveal the essential requirement of ferrichrome biosynthesis to promote outgrowth, as well as the necessity to take up ferrichrome from an external source via Str1 when ferrichrome biosynthesis is blocked.
During fungal spore germination, a resting spore returns to a conventional mode of cell division and resumes vegetative growth, but the requirements for spore germination are incompletely understood. Here, we show that copper is essential for spore germination in Germinating spores develop a single germ tube that emerges from the outer spore wall in a process called outgrowth. Under low-copper conditions, the copper transporters Ctr4 and Ctr5 are maximally expressed at the onset of outgrowth. In the case of Ctr6, its expression is broader, taking place before and during outgrowth. Spores lacking Ctr4, Ctr5, and the copper sensor Cuf1 exhibit complete germination arrest at outgrowth. In contrast, deletion only partially interferes with formation of outgrowing spores. At outgrowth, Ctr4-GFP and Ctr5-Cherry first co-localize at the spore contour, followed by re-location to a middle peripheral spore region. Subsequently, they move away from the spore body to occupy the periphery of the nascent cell. After breaking of spore dormancy, Ctr6 localizes to the vacuole membranes that are enriched in the spore body relative to the germ tube. Using a copper-binding tracker, results showed that labile copper is preferentially localized to the spore body. Further analysis showed that Ctr4 and Ctr6 are required for copper-dependent activation of the superoxide dismutase 1 (SOD1) during spore germination. This activation is critical because the loss of SOD1 activity blocked spore germination at outgrowth. Taken together, these results indicate that cell-surface copper transporters and SOD1 are required for completion of the spore germination program.
Mfc1 is a meiosis-specific protein that mediates copper transport during the meiotic program in Schizosaccharomyces pombe.
Although the mfc1؉ gene is induced at the transcriptional level in response to copper deprivation, the molecular determinants that are required for its copper starvation-dependent induction are unknown. Promoter deletion and site-directed mutagenesis have allowed identification of a new cis-regulatory element in the promoter region of the mfc1 ؉ gene. This cis-acting regulatory sequence containing the sequence TCGGCG is responsible for transcriptional activation of mfc1 ؉ under low-copper conditions. The TCGGCG sequence contains a CGG triplet known to serve as a binding site for members of the Zn (2) Cys (
The biophysical properties of the cytoplasm are major determinants of key cellular processes and adaptation. Many yeasts produce dormant spores that can withstand extreme conditions. We show that spores of Saccharomyces cerevisiae exhibit extraordinary biophysical properties, including a highly viscous and acidic cytosol. These conditions alter the solubility of more than 100 proteins such as metabolic enzymes that become more soluble as spores transit to active cell proliferation upon nutrient repletion. A key regulator of this transition is the heat shock protein, Hsp42, which shows transient solubilization and phosphorylation, and is essential for the transformation of the cytoplasm during germination. Germinating spores therefore return to growth through the dissolution of protein assemblies, orchestrated in part by Hsp42 activity. The modulation of spores’ molecular properties are likely key adaptive features of their exceptional survival capacities.
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