Question

E. coli cells with a mutated Z gene of the lac operon cannot grow in medium containing only lactose as the source of carbon because

• A. They cannot synthesize functional $\beta$-galactosidase.
• B. They cannot transport lactose from the medium into the cell.
• C. The lac operon is constitutively active in these cells.
• D. In the presence of glucose, E.coli cells do not utilize lactose.

Answer 1

The correct option is A They cannot synthesize functional $\beta$-galactosidase.

The lac operon is an operon responsible for transport and metabolism of lactose in Escherichia coli and other bacteria. It has three structural genes. The lac Z gene codes for $\beta$-galactosidase which converts lactose into glucose and galactose. Thus, E. coli cells with a mutated Z gene of the lac operon cannot grow in medium containing only lactose as the source of energy because they cannot synthesize functional $\beta$-galactosidase.

The lac Y gene that codes for permease which transports lactose into the cell. The lac A gene codes for transacetylase. A constitutive mutant produces the gene product continually and lack the control over its expression. In these mutants, the structural genes produced enzymes all the time in comparison to the wild type where the enzymes were synthesized only in the presence of lactose. Hence, they should have mutation in a gene other than those responsible for the structural genes. E. coli cells prefer glucose over any other carbon source; in presence of glucose, lac operon is switched off by catabolic repression which is not governed by lac Z gene. Thus, there is no effect of mutated lac Z gene on choice of not using lactose in presence of glucose. Option A is correct answer.