Question

Suppose that the E. coli gene for the lac operon repressor has a mutation that makes the repressor ineffective. What effect would this have on the bacterium's ability to catabolize lactose? Would the mutant strain have an advantage over wild-type cells? Explain your answer.

Short answer

The ineffective repressor would mean constant transcription of lactose digestion genes making the bacterium always primed for lactose breakdown. This might be advantageous in lactose-rich environments. However, it could be disadvantageous in scarce conditions due to unnecessary energy and resource consumption, providing an edge to wild-type cells in such cases.

Step-by-step solution

Understanding the Lac Operon

The lac operon is a set of genes in bacteria that is responsible for the breakdown of lactose into glucose. The lac operon repressor, on the other hand, prevents transcription of these genes, unless lactose is present to act as an inducer. If the repressor is ineffective, these genes would be constantly transcribed.

Effects on Lactose Catabolism

Since the lactose-digesting genes would continuously be transcribed, the bacterium would constantly be in a state of readiness to break down lactose. However, if no lactose is available, the energy and resources expended into the futile transcription and translation would not result in any benefit.

Advantages or Disadvantages over Wild-Type Cells

For the mutant strain, an advantage would be the promptness of response to the presence of lactose in the environment, as no time is needed to initiate the transcription of the required genes. However, an excess amount of energy and resources would be consumed, which could be disadvantageous in conditions where resources are scarce and need to be optimally utilized, hence giving an advantage to wild-type cells under such circumstances.

Key concepts

Lactose Catabolism

In order to understand the implications of a mutation in the lac operon repressor, it's essential to first delve into the process of lactose catabolism. Lactose catabolism is the metabolic process by which bacteria such as E. coli break down lactose, a disaccharide sugar composed of galactose and glucose, into its simple sugar components. This process not only provides energy but also yields essential metabolic intermediates for the bacteria.

Normally, this catabolic pathway is carefully regulated. Bacteria encode enzymes like beta-galactosidase that are used in this process. However, these enzymes are typically not produced when lactose is absent, because their production is needless and would waste valuable biological resources. If the repressor of the lac operon is mutated and ineffective, as mentioned in the exercise, the bacterium would constantly produce the enzymes required for lactose catabolism, even when lactose is not present. This leads to an unregulated, energy-intensive state that could be wasteful for the cell.

Gene Transcription

The concept of gene transcription in bacteria hinges on the ability of the organisms to produce RNA copies from their DNA, which go on to direct protein synthesis. It's the crucial step where the genetic code becomes the functional proteins that carry out life processes.

The lac operon repressor specifically interacts with DNA to prevent transcription when lactose is not available. When lactose becomes available, it binds to the repressor, causing a conformational change that allows transcription to proceed. With a mutation rendering the repressor ineffective, gene transcription for lactose digestion proteins becomes unrestrained. This anomaly would mean that the cellular machinery would be in continuous operation transcribing genes for enzymes like beta-galactosidase, even without the natural inducer, lactose, resulting in unnecessary energy and resource expenditure.

Bacterial Gene Regulation

Bacterial gene regulation ensures that gene expression is responsive to environmental changes, such as nutrient availability. The lac operon is a classic model for understanding bacterial gene regulation. It includes a cluster of genes required for lactose utilisation that are regulated in a coordinated manner by the lac repressor protein.

Under normal conditions, when lactose is scarce, the repressor binds to the operator site of the lac operon, obstructing RNA polymerase and thus preventing transcription. This tight regulation conserves bacterial resources and displays a finely tuned response to environmental cues. The mentioned mutation would compromise this regulatory mechanism, leading to a constitutively active set of genes, disregarding the actual need for their protein products, and thereby challenging the cell's homeostasis and efficiency.

Mutant Strain Advantages

A mutant strain of E. coli with an ineffective lac repressor could have certain environmental advantages. In fluctuating environments where lactose availability is unpredictable, the mutant strain would be able to immediately begin breaking down lactose without delay, since the catabolic enzymes would already be present. This could provide a critical advantage for survival and growth when competing with other microbes.

However, it must be noted that these advantages come with a significant trade-off. In the absence of lactose, the mutation results in a continuous drain of cellular resources, producing enzymes that serve no function. This poses a disadvantage in resource-limited conditions, as wild-type cells would have the upper hand by not squandering resources on unnecessary protein synthesis. The impact of the mutation would thus be context-dependent, offering benefits in some environments while creating setbacks in others.