Identification and Functional Analysis of the erh1 (+) Gene Encoding Enhancer of Rudimentary Homolog from the Fission Yeast Schizosaccharomyces pombe

The ERH gene encodes a highly conserved small nuclear protein with a unique amino acid sequence and three-dimensional structure but unknown function. The gene is present in animals, plants, and protists but to date has only been found in few fungi. Here we report that ERH homologs are also present in all four species from the genus Schizosaccharomyces, S. pombe, S. octosporus, S. cryophilus, and S. japonicus, which, however, are an exception in this respect among Ascomycota and Basidiomycota. The ERH protein sequence is moderately conserved within the genus (58% identity between S. pombe and S. japonicus), but the intron-rich genes have almost identical intron-exon organizations in all four species. In S. pombe, erh1(+) is expressed at a roughly constant level during vegetative growth and adaptation to unfavorable conditions such as nutrient limitation and hyperosmotic stress caused by sorbitol. Erh1p localizes preferentially to the nucleus with the exception of the nucleolus, but is also present in the cytoplasm. Cells lacking erh1(+) have an aberrant cell morphology and a comma-like shape when cultured to the stationary phase, and exhibit a delayed recovery from this phase followed by slower growth. Loss of erh1(+) in an auxotrophic background results in enhanced arrest in the G1 phase following nutritional stress, and also leads to hypersensitivity to agents inducing hyperosmotic stress (sorbitol), inhibiting DNA replication (hydroxyurea), and destabilizing the plasma membrane (SDS); this hypersensitivity can be abolished by expression of S. pombe erh1(+) and, to a lesser extent, S. japonicus erh1(+) or human ERH. Erh1p fails to interact with the human Ciz1 and PDIP46/SKAR proteins, known molecular partners of human ERH. Our data suggest that in Schizosaccharomyces sp. erh1(+) is non-essential for normal growth and Erh1p could play a role in response to adverse environmental conditions and in cell cycle regulation.