Q. 80 E. coli requires three genes, galK (kinase), galT (transacetylase) and galE (epimerase) to utilize galactose. If there is a mutation in any one of these genes, the mutant cannot utilize galactose. Which one of the following combinations of merodiploids will support the growth of mutants on galactose?
(P) galK+galT+galE–/ galK–galT+galE–
(Q) galK–galT+ +galE–/galK+galT–galE+
(R) galK+galT–galE–/galK–galT–galE+
(S) galK+galT+galE–/galK+galT–galE+
(A) P & Q (B) P & R (C) R & S (D) Q & S
Solving E. coli Galactose Utilization Merodiploid Genetics Problem
This article breaks down a classic genetics question on E. coli galactose metabolism, explaining merodiploids, complementation, and why only specific combinations restore growth on galactose as sole carbon source.
Galactose Metabolism in E. coli
E. coli metabolizes galactose through the Leloir pathway, requiring three enzymes encoded by the gal operon:
- galK: Galactokinase, phosphorylates galactose to galactose-1-phosphate.
- galT: Galactose-1-phosphate transacetylase (uridylyltransferase), transfers UDP from UDP-glucose.
- galE: UDP-galactose-4-epimerase, interconverts UDP-galactose and UDP-glucose.
A mutation in any gene (galK–, galT–, or galE–) blocks the pathway, preventing growth on galactose minimal medium. Merodiploids (partial diploids, F’ plasmid/chromosome) test complementation: wild-type allele on one copy supplies functional enzyme missing on the other.
Growth occurs only if all three enzymes are present across both genomes.
Decoding Merodiploid Notation
Notation: galK galT galE / galK galT galE
- First set: F’ plasmid genes.
- Second set: Chromosome genes.
- + = wild-type (functional); – = mutant (nonfunctional).
Analysis of Each Option
Option P: galK+ galT+ galE– / galK– galT+ galE–
- Plasmid: galK+ (✓), galT+ (✓), galE– (✗).
- Chromosome: galK– (✗), galT+ (✓), galE– (✗).
- Combined: galK+, galT+, galE absent → No growth.
Option Q: galK– galT+ galE– / galK+ galT– galE+
- Plasmid: galK– (✗), galT+ (✓), galE– (✗).
- Chromosome: galK+ (✓), galT– (✗), galE+ (✓).
- Combined: galK+, galT+, galE+ → All enzymes present; growth supported.
Option R: galK+ galT– galE– / galK– galT– galE+
- Plasmid: galK+ (✓), galT– (✗), galE– (✗).
- Chromosome: galK– (✗), galT– (✗), galE+ (✓).
- Combined: galK+, galT absent, galE+ → No growth.
Option S: galK+ galT+ galE– / galK+ galT– galE+
- Plasmid: galK+ (✓), galT+ (✓), galE– (✗).
- Chromosome: galK+ (✓), galT– (✗), galE+ (✓).
- Combined: galK+, galT+, galE+ → All enzymes present; growth supported.
Correct Answer: (D) Q & S
Options Q and S provide complementation for all three genes via trans-acting diffusible enzymes. P and R fail due to missing galE or galT.
| Option | galK | galT | galE | Growth? |
|---|---|---|---|---|
| P | ✓ | ✓ | ✗ | No |
| Q | ✓ | ✓ | ✓ | Yes |
| R | ✓ | ✗ | ✓ | No |
| S | ✓ | ✓ | ✓ | Yes |
This principle applies to dominance tests in bacterial genetics, key for understanding operon function and complementation groups.
