The Saccharomyces cerevisiae ACP2 gene encodes an essential HMG1-like protein

The high-mobility-group (HMG) proteins, a group of nonhistone chromatin-associated proteins, have been extensively characterized in higher eucaryotic cells. To test the biological function of an HMG protein, we have cloned and mutagenized a gene encoding an HMG-like protein from the yeast Saccharomyces cerevisiae. A yeast genomic DNA library was screened with an oligonucleotide designed to hybridize to any yeast gene containing an amino acid sequence conserved in several higher eucaryotic HMG proteins. DNA sequencing and Northern (RNA) blot analysis revealed that one gene, called ACP2 (acidic protein 2), synthesizes a poly(A)+ RNA in S. cerevisiae which encodes a 27,000-molecular-weight protein whose amino acid sequence is homologous to those of calf HMG1 and HMG2 and trout HMGT proteins. Standard procedures were used to construct a diploid yeast strain in which one copy of the ACP2 gene was mutated by replacement with the URA3 gene. When this diploid was sporulated and dissected, only half of the spores were viable. About half of the nonviable spores proceeded through two or three cell divisions and then stopped dividing; the rest did not germinate at all. None of the viable spores contained the mutant ACP2 gene, thus proving that the protein encoded by ACP2 is required for cell viability. The results presented here demonstrate that an HMG-like protein has an essential physiological function.