Regulatory subunit of the CDC7-DBF4 kinase, also called DBF4-dependent kinase (DDK), which is involved in cell cycle regulation of premitotic and premeiotic chromosome replication and in chromosome segregation. DDK plays an essential role in initiating DNA replication at replication origins by phosphorylating the MCM2 and MCM4 subunits of the MCM2-7 helicase complex. DBF4 recruits the catalytic subunit CDC7 to MCM2 and to origins of replication. DDK has also postreplicative functions in meiosis. DDK phosphorylates the meiosis-specific double-strand break protein MER2 for initiation of meiotic recombination. Interacts with CDC5 during meiosis to promote double-strand breaks and monopolar spindle orientation. Inhibits CDC5 activity during mitosis through direct binding to its PBD. Heterodimer with CDC7 to form the DBF4-dependent kinase (DDK) complex. Interacts (via PBD-binding motif) with CDC5 (via POLO box domains). Interacts (via N-terminus) with ORC2, ORC3 and RAD53. Binds to ARS1 origin DNA. Cell cycle-regulated. Protein levels increase as cells begin S phase and remain high through late mitosis. Phosphorylated by CDC7 and by CDC5.