Many of the gene products involved in DNA synthesis were initially defined by studying mutant E. coli strains that could not synthesize DNA.
The dnaE gene encodes the α subunit of DNA polymerase III. What effect is expected from a mutation in this gene? How could the mutant strain be maintained?

Verified step by step guidance

1. Understand the role of the dnaE gene: It encodes the α subunit of DNA polymerase III, which is the main enzyme responsible for the synthesis of the leading and lagging DNA strands during replication in E. coli.
2. Predict the effect of a mutation in dnaE: Since the α subunit has the polymerase activity, a mutation that disrupts its function would likely impair or completely block DNA synthesis, leading to a lethal phenotype because the cell cannot replicate its DNA.
3. Consider how the mutant strain could be maintained despite the lethal effect: One common method is to provide a functional copy of the dnaE gene on a plasmid (a small, circular DNA molecule) that complements the defective chromosomal gene, allowing the cell to survive and replicate.
4. Alternatively, maintain the mutant strain under permissive conditions if the mutation is temperature-sensitive, meaning the mutant protein functions at a lower (permissive) temperature but not at a higher (restrictive) temperature.
5. Summarize that the mutation in dnaE causes defective DNA replication, and the mutant strain can be maintained by genetic complementation or conditional growth conditions that allow survival despite the mutation.

Key Concepts

Role of DNA Polymerase III in DNA Synthesis

DNA polymerase III is the primary enzyme responsible for bacterial DNA replication, synthesizing the new DNA strand by adding nucleotides complementary to the template. The α subunit, encoded by dnaE, carries the polymerase activity essential for chain elongation. A mutation in dnaE can impair DNA replication, leading to defective or halted DNA synthesis.

Effects of Mutations in Essential Genes

Mutations in essential genes like dnaE often result in lethal or severely growth-impaired phenotypes because the gene product is critical for survival. Such mutations can cause loss of function or dominant negative effects, preventing normal cellular processes like DNA replication, which is vital for cell division and viability.

Maintenance of Mutant Strains via Complementation or Conditional Mutations

Mutant strains with lethal mutations can be maintained using complementation with a plasmid carrying the wild-type gene or by using conditional mutations (e.g., temperature-sensitive alleles) that allow normal function under permissive conditions. This enables growth and study of mutants under controlled environments despite the essential gene defect.