In the absence of lacY permease, which strain cannot grow on a medium containing lactose as the sole carbon source at 42°C?
Correct answer: A. lac Z-
The lacZ gene encodes β-galactosidase, which is necessary for the breakdown of lactose. Without lacY permease, lactose cannot enter the cell and thus, cells lacking lacZ will not be able to utilize lactose as a carbon source at 42°C.