Synthesis of the amino acid histidine is a multistep anabolic pathway that uses the products of 13 genes (hisA to hisM) in E. coli. Two independently isolated  E. coli mutants, designated his1⁻ and his2⁻ are studied in a conjugation experiment. A his⁺ F' donor strain that carries a copy of the hisJ gene on the plasmid is mated with a his1⁻ recipient strain in Experiment 1 and with a his2⁻ recipient in Experiment 2. The exconjugants are grown on plates lacking histidine. Growth is observed among the exconjugants of Experiment 2 but not among those of Experiment 1.

What is the genotype of exconjugants in Experiment 2?

Key Concepts

Gene Function and Pathways

In genetics, genes encode proteins that perform specific functions in biological pathways. The synthesis of amino acids, such as histidine, involves multiple genes working together in a coordinated manner. Understanding how these genes interact and contribute to metabolic pathways is crucial for analyzing genetic mutations and their effects on organismal growth.

Conjugation in Bacteria

Conjugation is a process of horizontal gene transfer in bacteria, where genetic material is transferred from one bacterium to another through direct contact. In this experiment, the his⁺ F' donor strain provides the hisJ gene to the his1⁻ and his2⁻ recipient strains. The success of this gene transfer can be assessed by the growth of exconjugants on histidine-deficient media, indicating whether the transferred gene restored the ability to synthesize histidine.

Mutations and Complementation

Mutations can lead to loss of function in genes, as seen with the his1⁻ and his2⁻ strains, which are unable to synthesize histidine. Complementation occurs when a mutant strain receives a functional copy of a gene from another source, allowing it to regain the lost function. The observation of growth in Experiment 2 suggests that the his2⁻ strain can complement the mutation with the hisJ gene from the donor, while the his1⁻ strain cannot, indicating a difference in the nature of the mutations.