S-phase feedback control in budding yeast independent of tyrosine phosphorylation of P^34cdc28

Abstract

IN somatic cells, entry into mitosis depends on the completion of DNA synthesis. This dependency is established by S-phase feed-back controls that arrest cell division when damaged or unreplicated DNA is present¹. In the fission yeast Schizosaccharomyces pombe, mutations that interfere with the phosphorylation of tyrosine 15 (Y15) of p^34cdc2, the protein kinase subunit of maturation promoting factor, accelerate the entry into mitosis and abolish the ability of unreplicated DNA to arrest cells in G2 (ref. 2). Because the tyrosine phosphorylation of p^34cdc2 is conserved in S. pombe³, Xenopus⁴, chicken⁵ and human⁶cells, the regulation of p^34cdc2-Y15 phosphorylation could be a universal mechanism mediating the S-phase feedback control and regulating the initiation of mitosis^7,8. We have investigated these phenomena in the budding yeast Saccharomyces cerevisiae. We report here that the CDC28 gene product (the S.cerevisiae homologue of cdc2) is phosphorylated on the equivalent tyrosine (Y19) during S phase but that mutations that prevent tyrosine phosphorylation do not lead to premature mitosis and do not abolish feedback controls. We have therefore demonstrated a mechanism that does not involve tyrosine phosphorylation of p³⁴ by which cells arrest their division in response to the presence of unreplicated or damaged DNA. We speculate that this mechanism may not involve the inactivation of p³⁴ catalytic activity.