Php4 Is a Key Player for Iron Economy in Meiotic and Sporulating Cells

Meiosis is essential for sexually reproducing organisms, including the fission yeast Schizosaccharomyces
pombe. In meiosis, chromosomes replicate once in a diploid precursor cell (zygote), and then
segregate twice to generate four haploid meiotic products, named spores in yeast. In S. pombe, Php4 is
responsible for the transcriptional repression capability of the heteromeric CCAAT-binding factor to negatively
regulate genes encoding iron-using proteins under low-iron conditions. Here, we show that the
CCAAT-regulatory subunit Php4 is required for normal progression of meiosis under iron-limiting conditions.
Cells lacking Php4 exhibit a meiotic arrest at metaphase I. Microscopic analyses of cells expressing
functional GFP-Php4 show that it colocalizes with chromosomal material at every stage of meiosis under low
concentrations of iron. In contrast, GFP-Php4 fluorescence signal is lost when cells undergo meiosis under
iron-replete conditions. Global gene expression analysis of meiotic cells using DNA microarrays identified
137 genes that are regulated in an iron- and Php4-dependent manner. Among them, 18 genes are
expressed exclusively during meiosis and constitute new putative Php4 target genes, which include
hry1+ and mug14+. Further analysis validates that Php4 is required for maximal and timely repression of
hry1+ and mug14+ genes. Using a chromatin immunoprecipitation approach, we show that Php4 specifically
associates with hry1+ and mug14+ promoters in vivo. Taken together, the results reveal that in iron-starved
meiotic cells, Php4 is essential for completion of the meiotic program since it participates in global gene
expression reprogramming to optimize the use of limited available iron.