Genetic Regulation and the Lactose Operon

ID #1411

In bacterial genetic regulation you mentioned that lac mRNA is degraded from the 3' to 5' end, which results in different amounts of protein products being made by the lacZ, lacY, and lacA genes. My question is while the mRNA is being degraded, is there some mechanism that prevents ribosomes from translating the lacA gene entirely, or are ribosomes still translating, but nonfunctional protein is coming out due to the degradation at the 3' end?

Good question. As a relevant side note, remember the short half-lives of bacterial mRNAs, where a given mRNA molecule is only going to be around for probably a couple minutes of real time. Given the time frame in question, if it were possible for the cell to recognize that the 3' end of the mRNA were "broken" (because it's been degraded) and respond by binding some secondary molecule to the RBS of that coding region, by the time that regulatory molecule got there, there probably wouldn't be anything left to bind to! So it's not really necessary to prevent translation of a coding region that no longer has the correct "end." The ribosomes will just be making a few copies of prematurely truncated protein, which will then be degraded. Plus, as you'll learn in MCB 250 (but do not need to know for our exam), it's possible for a single ribosome to plow right through from one coding region in a polycistronic mRNA to the next. It's not making a polyprotein, it is making distinct protein products, but it does not have to dissociate and reinitiate, rendering the type of regulation you asked about unnecessary.

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