Pol12, the B subunit of DNA polymerase alpha, functions in both telomere capping and length regulation.

The regulation of telomerase action, and its coordination with conventional DNA replication and chromosome end "capping," are still poorly understood. Here we describe a genetic screen in yeast for mutants with relaxed telomere length regulation, and the identification of Pol12, the B subunit of the DNA polymerase alpha (Pol1)-primase complex, as a new factor involved in this process. Unlike many POL1 and POL12 mutations, which also cause telomere elongation, the pol12-216 mutation described here does not lead to either reduced Pol1 function, increased telomeric single-stranded DNA, or a reduction in telomeric gene silencing. Instead, and again unlike mutations affecting POL1, pol12-216 is lethal in combination with a mutation in the telomere end-binding and capping protein Stn1. Significantly, Pol12 and Stn1 interact in both two-hybrid and biochemical assays, and their synthetic-lethal interaction appears to be caused, at least in part, by a loss of telomere capping. These data reveal a novel function for Pol12 and a new connection between DNA polymerase alpha and Stn1. We propose that Pol12, together with Stn1, plays a key role in linking telomerase action with the completion of lagging strand synthesis, and in a regulatory step required for telomere capping.