Mutation tolerance affected by environmental stresses in cells

[John Jacobus <crispy_bird@YAHOO.COM>]

I received a notice about this article through another

list server and thought I would pass it along.  Simply

stated, environmental stress, such as radiation

(?), will influence cellular ability to "tolerance"

deleterious mutations that would normally affect

unstressed cells.  Of course, there are limits to the

effects and the cells already already have a

mutational load that restricts colony growth.  It

appears that the colony sizes/growth rates for the

wild gets worse, and those factors for mutated cells,

which have a large deficit in growth, get better with

increasing stress level.  Both a wild type and mutated

strain of E. coli were exposed to two types of

environmental stresses: 



"The first class includes the bacteriostatic

antibiotics chloramphenicol and trimethoprim, which

specifically target translation and folic acid

biosynthesis, respectively. The second class includes

low pH, low temperature, high osmolarity and the

reducing reagent dithiothreitol, which are stresses

with wider impacts (the reducing reagent

dithiothreitol may have general impacts on protein

disulfide bonds as well as more specific impacts on

modules involved in maintaining redox balance."







----------------------------------------------------------



"Environmental stresses can alleviate the average

deleterious effect of mutations"

by Roy Kishony and Stanislas Leibler

Laboratory of Living Matter, Rockefeller University,

1230 York Avenue, New York, NY 10021, USA



Journal of Biology 2003 2(2):14 (published

29 May 2003)



Abstract 





Background



Fundamental questions in evolutionary genetics,

including the possible advantage of sexual

reproduction, depend critically on the effects of

deleterious mutations on fitness. Limited existing

experimental evidence suggests that, on average, such

effects tend to be aggravated under environmental

stresses, consistent with the perception that stress

diminishes the organism's ability to tolerate

deleterious mutations. Here, we ask whether there are

also stresses with the opposite influence, under which

the organism becomes more tolerant to mutations.



Results



We developed a technique, based on bioluminescence,

which allows accurate automated measurements of

bacterial growth rates at very low cell densities.

Using this system, we measured growth rates of

Escherichia coli mutants under a diverse set of

environmental stresses. In contrast to the perception

that stress always reduces the organism's ability to

tolerate mutations, our measurements identified

stresses that do the opposite – that is, despite

decreasing wild-type growth, they alleviate, on

average, the effect of deleterious mutations.



Conclusions



Our results show a qualitative difference between

various environmental stresses ranging from

alleviation to aggravation of the average effect of

mutations. We further show how the existence of

stresses that are biased towards alleviation of the

effects of mutations may imply the existence of

average epistatic interactions between mutations. The

results thus offer a connection between the two main

factors controlling the effects of deleterious

mutations: environmental conditions and epistatic

interactions.



----------------------





=====

-- John

John Jacobus, MS

Certified Health Physicist

e-mail:  crispy_bird@yahoo.com



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