Question

Compare the typical lifetimes of mRNA and proteins in a bacterial, yeast and human cells and describe how these lifetimes interact with the length of the cell cycle in these different organisms.

Answer 1

Messenger RNA is not the only target of degradation. Protein molecules are themselves also the target of specific destruction, though generally, their lifetimes tend to be longer than the mRNAs that lead to their synthesis, as discussed below. Because of these long lifetimes, under fast growth rates the number of copies of a particular protein per cell is reduced not because of an active degradation process, but simply because the cell doubles all its other constituents and divides into two daughters leaving each of the daughters with half as many copies of the protein of interest as were present in the mother cell. To understand the dilution effect, imagine that all protein synthesis for a given protein has been turned off while the cell keeps on doubling its volume and shortly thereafter divides. In terms of absolute values, if the number of copies of our protein of interest before division is N, afterwards it is N/2. In terms of concentrations, if it started with a concentration c, during the cell cycle it got diluted to c/2 by the doubling of the volume. This mechanism is especially relevant in the context of bacteria where the protein lifetimes are often dominated by the cell division time. As a result, the total protein loss rate α (the term carrying the same meaning as γ for mRNA) is the sum of a part due to active degradation and a part due to the dilution that occurs when cells divide and we can write the total removal rate in the form α=αactive+αdilution.