How environmental factors regulate mutagenesis and gene transfer in microorganisms |
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Authors: | Vassili V Velkov |
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Institution: | (1) Institute of Biochemistry and Physiology of Microorganisms, Russian Academy of Sciences, 142292 Pushchino, Moscow Region, Russia |
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Abstract: | This review is focused on the physiological and evolutionary strategies of the processes occurring during the entry of microbial
cells into stationary phase and the subsequent period of stasis. The molecular mechanisms adapting microorganisms from exponential
growth to a static state involve activation and complex regulation of the stationary factor Sigma-S, which directs RNA polymerase
to the specific promoters. As a result the static cells acquire general resistance (simultaneous tolerances) to different
environmental stresses. In parallel with the physiological adaptation to stasis, diverse genetical processes are aimed towards
resuming the growth of the static cells. Different types of mutagenesis occur: (i) in cells entering stasis and (ii) in static
cells (adaptive mutagenesis). Cessation of growth induces the transient hypermutator state resulting in the accumulation of
random mutations in the subpopulation of the static cells. If by chance, one or a few of such mutations lead to resumption
of division, the growing cell will return to a normal mechanism of spontaneous mutagenesis.
Another mechanism for generating genetical variability in stressed cells involves transposons and conjugative plasmids. Stresses
can stimulate the excision of some transposons, which, in turn, can generate chromosomal mutations and activate intracellular
mechanisms of mutagenesis. Under stress some conjugative plasmids activate genes encoding antirestriction proteins that repress
restriction-modification systems of the recipient cells. Moreover, under stress special cellular mechanisms decrease (alleviate)
the activity of restriction-modification systems which, in turn, enhance the probability of gene transfer into the stressed
cells.
Under stress, the efficiency of inter-species genetical barriers also decreases. This, stimulates inter-species gene transfer
and may lead to a burst of incipient speciation in the population of non-growing cells. After resumption of growth the genetical
barriers leading to isolation will be restored.
In general, the cessation of growth “switches on”, and resumption of growth “switches off”, a set of special processes that
are responsible for generating bursts of genetical variability in populations of microorganisms.
This article is dedicated to the memory of Nikolai V Timofeev-Ressovsky (1900–1981). |
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Keywords: | Microorganisms stationary phase Sigma-S general resistance adaptive mutagenesis inter-species gene transfer speciation evolution |
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