Question

Your lab has identified a mutant form of salmonella in which the lac operon is rendered insensitive to the presence of glucose. On the other hand, other genes and operons in this mutant salmonella are still controlled normally by glucose. Provide a plausible explanation for where the mutation is likely located.

Answer 1

The situation given here is such that the identified bacterial (salmonella) mutant is insensitive to the presence of glucose. Let us understand the role of glucose in the operation of the lac operon, first. We know that the Lac operon is an inducible operon where the operon is switched off usually but then a small molecule comes and switches on this particular operon (lactose acts as this small molecule here).

Now we also know that organisms usually prefer glucose over lactose if both the sugars are present in the medium to utilize them as their carbon source (that is because glucose requires fewer steps to be metabolized and get converted to energy). So, during this (times where both the sugars, glucose, and lactose are present in the medium), these organisms have to keep other metabolic machinery shut (such as lac operon) and only after complete utilization of this glucose they will move on to using lactose as their carbon source. Hence, during the presence of glucose, these organisms will switch off the lac operon via a mechanism called catabolite repression.

Remember that the operon has a lot of sites (operator, promoter, etcetera) some acts as enhancers and some acts as repressors of the operon (we won't get into the details here but since the center of the discussion is the effect of glucose on the lac operon we will talk about how glucose governs the induction of the lac operon)

Here in catabolite repression, when there would be less amount of cyclic AMP or cAMP (adenosine monophosphate) synthesized (usually in the presence of a higher amount of glucose). This cAMP will bind to yet another protein called the Catabolite activator protein/CAP protein, the cAMP-CAP complex will then bind the CAP site present near the promoter region of the lac operon.

This complex (cAMP-CAP) bound with the DNA physically interacts with the RNA polymerase and thus aids in the starting of the transcription of the lac operon genes (it actually causes an increment in the binding affinity of the RNA polymerase with the lac operon). The CAP protein is synthesized by a gene called the crp gene if this gene is mutated there will not be a functional CAP protein formed, in that case, the lac operon will be rendered insensitive (inactive due to no functional cAMP-CAP formed and which is able to bind the CAP site to start transcription of the operon)

If a mutant is identified which is insensitive to the presence of glucose in salmonella isolate then it is highly likely that the mutation is in the CAP protein producing gene (crp gene) which is causing non functional/mutant CAP protein which might not bind with the cAMP further non interaction of this complex with CAP sites will cause inactivation of the lac operon. This way the lac operon has become insensitive that it is not reacting to/sensing the glucose levels and getting induced.

This inference could be further cross-checked by realizing the fact given in the situation that other operons and genes are responding to glucose.