The fission yeast homologue of Orc4p binds to replication origin DNA via multiple AT-hooks

The origin recognition complex (ORC) was originally identified in the yeast Saccharomyces cerevisiae as a protein that specifically binds to origins o f DNA replication. Although ORC appears to play an essential role in the in itiation of DNA replication in the tells of all eukaryotes, its interaction s with DNA have not been defined in species other than budding I-east. We h ave characterized a Schizosaccharomyces pombe homologue of the ORC subunit, Orc4p. The homologue (Orp4p) consists of two distinct functional domains. The C-terminal domain shows strong sequence similarity to human, frog, and yeast Orc4 proteins, including conserved ATP-binding motifs, The N terminal domain contains nine copies of the AT-hook motif found in a number of DNB- binding proteins, including the members of the HMG-I(Y) family of chromatin proteins. AT-hook motifs are known from biochemical and structural studies to mediate binding to the minor groove of AT-tracts in DNA. Orp4p is essen tial for viability of Sc. pombe and is expressed throughout the cell cycle, The Orp4 protein (and its isolated N-terminal domain) binds to the Sc. pom be replication origin, ars1. The DNA binding properties of Orp4p provide a plausible explanation for the characteristic features of Sc. pombe origins of replication, which differ significantly from those of Sir, cerevisiae.