Emerging evidence suggests that Cdc13-Stn1-Ten1 (CST), an RPA-like ssDNA-binding complex, may regulate primase-Pol α (PP) activity at telomeres constitutively, and at other genomic locations under conditions of replication stress. Here we examine the mechanisms of PP stimulation by CST using purified complexes derived from Candida glabrata. While CST does not enhance isolated DNA polymerase activity, it substantially augments both primase activity and primase-to-polymerase switching. CST also simultaneously shortens the RNA and lengthens the DNA in the chimeric products. Stn1, the most conserved subunit of CST, is alone capable of PP stimulation. Both the N-terminal OB fold and the C-terminal winged-helix domains of Stn1 can bind to the Pol12 subunit of the PP complex, and stimulate PP activity. Our findings provide mechanistic insights on a well-conserved pathway of PP regulation that is critical for genome stability.

Background: Cdc13 in Candida species is small and missing conserved domains. Results: A second Cdc13 homologue (Cdc13B) was shown to form heterodimers with Cdc13 and to mediate overlapping functions in telomere protection. Conclusion: Cdc13 has undergone duplication and functional specialization in a branch of Saccharomycotina yeast. Significance: Understanding the assembly and mechanisms of fungal CST complex provides insights on the evolution and function of this crucial telomere complex.