Mec1p and Tel1p are required for HU-induced Rfa1p phosphorylation. (A) Western blot analysis of Rfa1p from exponentially growing (–) or HU-treated (+) cells. Strains employed were: TWY397 (wt), TWY308 (mec1), TWY312 [rad53 (= mec2)], TWY316 (mec3) and TWY398 (rad9). P-Rfa1p, phosphorylated form of Rfa1p. (B) YDM937 (mec1-1 tel1Δ1) cells transformed with pRS316 (vector), pRS316 containing TEL1 (pTEL1) or pRS316 containing MEC1 (pMEC1) were analyzed as in (A). *, immunoreactive band that migrates faster than full-length Rfa1p and is presumed to be an Rfa1p degradation product. The intensity of this band varies between experiments.