Examples of prokaryotic genetic programs
detrimental to individual cell
Antimicrobial Discovery Center and Department of Biology, Northeastern University, Boston
Programs that replicate more efficiently are selected for, and this also applies to cases when the program appears to be counter-productive, at least to an individual. We then consider how such individually counterproductive programs may benefit a population.
Perhaps the most wide-spread program that limits the reproduction of individual cells is one that leads to the formation of dormant persister cells. In all bacterial species, a small subpopulation of persisters plays the role of a safeguard against threats. Persisters are formed in response to increased expression of toxin/antitoxin proteins such as HipA, a kinase of Ef-Tu, orTisB, a membrane-acting peptide. Persisters are highly tolerant to antibiotics and other lethal factors, but this comes at a price of forfeiting propagation. In the case of persisters, at least in the presence of a lethal factor, there is direct individual benefit. This is not so obvious in a seemingly bizarre strategy of "unculturable" microorganisms that actually make up the vast majority, 99% of all bacterial species. Unculturable bacteria in a marine sediment biofilm evolved to lose their genes for making siderophores, iron chelators. They now depend stringently on the presence of neighboring culturable bacteria. Their growth is then restricted to familiar environments, and they have lost the ability to colonize new territory.
Programmed cell death is one of the more striking examples of counter-productive behavior, and in bacteria, formation of a fruiting body by Myxococcus depends on the lysis of members of the community, which provides nutrients to the rest of the population. Apart from this isolated example, a more widespread PCD occurs during formation of biofilms by a number of species, where cells lyse and release DNA, which serves to physically bind together and strengthen the community. Finally, PCD occurs during DNA damage, when expression of TisB causes part of the population to differentiate into persisters, and the same TisB peptide kills about half of the cells. It appears that cells which are less fit are not able to sustain damage caused by TisB and dye, a possible case of phenoptosis aimed at clearing the population of individuals that will ultimately be unable to repair, and will only draw resources from their healthier kin.
Homo Sapiens Liberatus Workshop, Moscow State University, May 2010