THE HUMAN HOMOLOG OF SACCHAROMYCES-CEREVISIAE CDC45

In budding yeast Saccharomyces cerevisiae CDC45 is an essential gene r equired for initiation of DNA replication. A structurally related prot ein Tsd2 is necessary for DNA replication in Ustilago maydis. We have identified and cloned the gene for a human protein homologous to the f ungal proteins. The human gene CDC45L is 30 kilobases long and contain s 15 introns, The 16 exons encode a protein of 566 amino acids. The hu man protein is 52 and 49.5% similar to CDC45p and Tsd2p, respectively. The level of CDC45L mRNA peaks at G(1)-S transition, but total protei n amount remains constant throughout the cell cycle. Consistent with a role of CDC45L protein in the initiation of DNA replication it co-imm unoprecipitates from cell extracts with a putative replication initiat or protein, human ORC2L, In addition, subcellular fractionation indica tes that the association of the protein with the nuclear fraction beco mes labile as S phase progresses. The CDC45L gene is located to chromo some 22q11.2 region by cytogenetics and by fluorescence in situ hybrid ization, This region, known as DiGeorge syndrome critical region, is a minimal area of 2 megabases, which is consistently deleted in DiGeorg e syndrome and related disorders. The syndrome is marked by parathyroi d hypoplasia, thymic aplasia, or hypoplasia and congenital cardiac abn ormalities, CDC45L is the first gene mapped to the DiGeorge syndrome c ritical region interval whose loss may negatively affect cell prolifer ation.