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.

Why is growth observed in Experiment 2 but not in Experiment 1?

Key Concepts

Gene Function and Pathway

The synthesis of histidine in E. coli involves a multistep anabolic pathway regulated by multiple genes. Each gene in the his operon contributes to a specific step in the histidine biosynthesis process. Understanding how these genes interact and their roles in the pathway is crucial for analyzing the growth of mutants in the presence of a donor strain.

Conjugation in Bacteria

Conjugation is a process of horizontal gene transfer in bacteria, where genetic material is exchanged between two bacterial cells through direct contact. In this experiment, the his⁺ F' donor strain provides a functional copy of the hisJ gene to the recipient strains. The success of this gene transfer and its impact on the growth of the exconjugants is essential to understanding the experimental outcomes.

Mutant Phenotypes

The his1⁻ and his2⁻ mutants represent different genetic alterations that affect histidine synthesis. The specific mutations in these strains determine their ability to utilize the functional gene provided by the donor strain. Analyzing the nature of these mutations helps explain why one mutant can grow in the absence of histidine while the other cannot, highlighting the importance of genetic context in metabolic pathways.