DNA replication occurs through a complex series of reactions that are mechanistically coordinated at the replication fork. Studies of model DNA replication systems indicate that origin recognition by an initiator protein permits assembly and activation of the replicative helicase at the origin. Helicases unwind duplex DNA to generate a single-strand template on which primases can initiate synthesis of RNA primers that are 4 -20 nucleotides in length and that are extended by DNA polymerases to make Okazaki fragments (1). Primases are physically coupled to the replicative DNA helicases and DNA polymerases, which translocate together through the duplex, resulting in DNA unwinding coincident with synthesis of RNA primers and their extension with deoxynucleotides.In eukaryotes, the Mcm proteins are essential replication factors that were identified as proteins required for minichromosomal maintenance in a genetic screen for mutants defective in initiation of replication (2). Six of these members are sequencerelated proteins, Mcm2-7, which interact to form a hexameric complex. Although a substantial body of data suggests that the Mcm2-7p complex acts as the replicative helicase (3), helicase activity has been detected only in the Mcm4-6-7 subcomplex (4 -6). Prior to the initiation of replication, the Mcm2-7 complex associates with the initiator at replication origins. At the G 1 /S transition, both S phase cyclin-dependent kinase and Cdc7p-Dbf4p kinase activities rise, promoting the maturation of the pre-replicative complex to the preinitiation complex (7). Phosphorylation of the chromatin-bound Mcm2-7 complex by the Cdc7-Dbf4 kinase (8) Previously, we reported that, in vitro, S. pombe Mcm10p (amino acids 1-593; SpMcm10p) binds preferentially to singlestranded DNA (ssDNA) and to the large subunit of the pol ␣-primase complex and activates its DNA synthetic activity (25). In addition, we found that Mcm10p facilitates the binding of the pol ␣-primase complex to primed DNA and forms a stable ternary complex, suggesting that Mcm10p recruits the pol ␣-primase complex to template DNA. In keeping with these in vitro findings, Mcm10p was shown recently to stabilize the large subunit of the pol ␣-primase complex in S. cerevisiae (15) as well as the chromatin association of the pol ␣-primase complex in S. pombe (26).In this study, we demonstrate that full-length SpMcm10p (amino acids 1-593) and its C-terminal fragment (amino acids 416 -593) contain primase activity that catalyzes the synthesis of oligoribonucleotides that are extended by Escherichia coli pol I. Primase activity both co-sedimented and co-eluted with these full-length and truncated proteins, although their hydro-* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.