A mutant strain of E. coli is grown in a culture medium containing lactose as the sole carbon source. It is observed that lactose efficiently enters the bacterial cells, but the cells are unable to hydrolyse the disaccharide into galactose and glucose. This indicates a missense mutation primarily in the:

Step-by-step solution

The lac operon consists of three structural genes: lacZ , lacY , and lacA . The lacZ gene codes for -galactosidase, which is responsible for the hydrolysis of the disaccharide lactose into its monomeric units, galactose and glucose. The lacY gene codes for permease, which increases the permeability of the cell to -galactosides (lactose), allowing it to enter the cell. Since lactose efficiently enters the bacterial cells, the lacY gene is functional. However, the inability of the cells to hydrolyse lactose indicates