You have isolated a strain of mutant yeast cells that divides normally at 30°C but cannot enter M phase at 37°C.You have isolated its mitotic cyclin and mitotic Cdk and find that both proteins are produced and can form a normal M-Cdk complex at both temperatures.Which of the following temperature-sensitive mutations could NOT be responsible for the behavior of this strain of yeast?
A)inactivation of a protein kinase that acts on the mitotic Cdk kinase B)inactivation of an enzyme that ubiquitylates M cyclin C)inactivation of a phosphatase that acts on the mitotic Cdk kinase D)a decrease in the levels of a transcriptional regulator required for producing sufficient amounts of M cyclin