Reader Comments

Re:The unpredictability of downstream effects

by Ann Gauger (2010-05-10)

In response to The unpredictability of downstream effects

Thanks for the comment. I agree that it is unexpected that a single base change in the second codon should shut down expression to such an extent. The base change is from AAA (lysine) to CAA (glutamine). Both amino acids have bulky side chains, but are oppositely charged. Because this change is to a truncated tet/trp D fusion protein, though, and is presumably non-functional, I would not expect the mutant protein as such have an effect on the expression of downstream genes.

The expression of the whole gene set (tet/trpD, trpCBA) is driven from the tet promoter. (Both trp operon promoters have been removed.) If I had to hazard a guess, I would say that the change in sequence from A to C has increased transcription termination for some reason. We haven't looked, though. And yes, the metabolic load is the result of expression of all these genes, not just trpA. My guess is that reducing the load by removing some of the genes from the plasmid, or by inserting them as single copies onto the chromosome would reduce the cost of expression proportionately.

As we said in the paper, this is not a situation likely to arise in nature. However, it does reflect the situation found in many laboratory studies of enzyme recruitment to new function. These studies rely on extreme over-expression and very strong positive selection to demonstrate recruitment. However, our results indicate that highly expressed genes encoding non-functional or even weakly functional enzymes are likely to be inactivated rather than improved in a natural bacterial population. This should be considered when evaluating the likelihood of such recruitment scenarios.



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