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 his^- 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?

Verified step by step guidance

1. Step 1: Understand the genetic basis of the problem. The synthesis of histidine involves multiple genes (hisA to hisM). Mutations in these genes can disrupt the pathway, leading to histidine auxotrophy (inability to synthesize histidine). The his1⁻ and his2⁻ mutants are defective in different genes within this pathway.
2. Step 2: Analyze the conjugation experiment setup. In Experiment 1, a his⁺ F' donor strain carrying the hisJ gene on a plasmid is mated with the his1⁻ recipient strain. In Experiment 2, the same his⁺ F' donor strain is mated with the his2⁻ recipient strain. The exconjugants are tested for their ability to grow on plates lacking histidine.
3. Step 3: Consider the role of the hisJ gene. The hisJ gene is one of the genes involved in histidine synthesis. If the his1⁻ mutant has a defect in a gene other than hisJ, the introduction of the hisJ gene via the plasmid will not restore the ability to synthesize histidine, and growth will not occur. This explains the lack of growth in Experiment 1.
4. Step 4: Examine the outcome of Experiment 2. If the his2⁻ mutant has a defect specifically in the hisJ gene, the introduction of the hisJ gene via the plasmid will complement the mutation, restoring the ability to synthesize histidine. This explains the observed growth in Experiment 2.
5. Step 5: Conclude the reasoning. Growth is observed in Experiment 2 because the hisJ gene provided by the plasmid complements the mutation in the his2⁻ strain. In Experiment 1, the hisJ gene does not complement the mutation in the his1⁻ strain, as the defect lies in a different gene within the histidine synthesis pathway.

Key Concepts

Gene Function and Pathway

The synthesis of histidine in E. coli involves a multistep anabolic pathway regulated by multiple genes. Each gene, from hisA to hisM, encodes enzymes that catalyze specific reactions in the pathway. Understanding how these genes interact and contribute to histidine production is crucial for analyzing the mutants' growth in the experiments.

Conjugation in Bacteria

Conjugation is a process of horizontal gene transfer in bacteria, where genetic material is transferred from a donor to a recipient cell through direct contact. In this scenario, the his⁺ F' donor strain provides the hisJ gene to the his1⁻ and his2⁻ mutants. The success of this transfer and the resulting phenotype depend on the compatibility of the genes involved in the recipient strains.

Mutant Phenotypes

The his1⁻ and his2⁻ mutants represent different genetic alterations affecting histidine synthesis. The observation of growth in Experiment 2 but not in Experiment 1 suggests that his2⁻ has a mutation that allows it to utilize the hisJ gene from the donor, while his1⁻ likely has a mutation that disrupts this ability. Analyzing the specific mutations helps explain the differential growth outcomes.