(A) GSTcdc18 allows the growth of cells depleted of Cdc18. The cdc18+ shut-off strain YMF15 carrying plasmids expressing GST, Cdc18, or GSTcdc18 under the control of the cdc18+ promoter and a vector control were plated on media with thiamine to turn off expression of the chromosomal cdc18+ (cdc18+ off) or media without thiamine as a control (cdc18+ on). (B) Purified GSTcdc18 is associated with a kinase. GST and GSTcdc18 were purified and proteins were separated on an SDS/polyacrylamide gel and stained with Coomassie brilliant blue (CBB, Left). The same fractions were immunoblotted and probed with antibodies against GST to unambiguously identify GST fusion proteins (anti-GST, Center). Purified GST and GSTcdc18 were assayed for kinase activity, using histone H1 as a substrate (kinase, Right). (C) Affinity-purified GSTcdc18 was fractionated on Mono Q, and fractions were assayed for the presence of GSTcdc18 by immunoblot analysis (anti-GST) and for H1 kinase activity (H1 kinase). Fraction numbers are indicated. 0, no protein control; A, affinity-purified GSTcdc18 fraction; L, Mono Q load.