A consortium of researchers, notably from CNRS, CEA, INRA and the Universities of Aix-Marseille (Université de la Méditerrané), Paris-Sud, Toulouse and Grenoble 1, have analysed the genome of the bacterium Ramlibacter tataouinensis TTB310, also known as the "desert bacterium." Decoding of the genome revealed the presence of kaiC, a gene with a function that had previously been found only in certain photosynthetic bacteria. In these bacteria, kaiC governs a molecular clock mechanism which regulates their cell cycle based on the alternation of day and night, in what is called an endogenous circadian rhythm . Furthermore, the results of the study suggest that this gene allows the "desert bacterium," which is non-photosynthetic, to coordinate its cell cycle with the water cycle in hot, dry deserts.
This discovery opens new avenues for research on the role of circadian molecular clocks in the environmental adaptation of both photosynthetic and non-photosynthetic bacteria. These results were published online on 2 September by the review PLoS ONE.
Hot, dry deserts are considered to be one of the most extreme environments, and thus the most challenging for life forms. The primary limiting factor for life is the severely limited water availability, since aside from rain episodes, the only water available is in the dew that appears at the end of the night. However, these deserts present other environmental constraints as well, such as their highly mineral nature (very little available organic matter), exposure to high levels of UV radiation, and extremely hot temperatures.
It was during studies on the bacteria responsible for alteration of the Tataouine meteorite in the desert of South Tunisia that researchers succeeded in isolating a new species of non-photosynthetic bacterium (Ramlibacter tataouinensis), of the betaproteobacteria group. This bacterium, ramlibacter tataouinensis TTB310, has an unusual cell cycle including two stages: one in which non-motile, desiccation-resistant cysts are produced, and a second in which motile rods responsible for dissemination are produced. Thus, it appears that, during the day, Ramlibacter tataouinensis TTB310 blocks its cell cycle in the cyst production stage in order to protect itself against heat and dryness. Only at the end of the night, when dew appears, does the bacterium trigger either cyst division or the second stage of its cycle -- in which the cysts divide into motile rods, and then revert to cyst form after dissemination (see diagram).
Genome analysis of Ramlibacter tataouinensis TTB310 first revealed the presence of a gene rarely identified in non-photosynthetic bacteria: the kaiC gene. The study then suggested that, in this bacterium, kaiC had a function similar to that found in various photosynthetic bacteria. Thus it appears that this gene acts as an endogenous molecular clock which allows Ramlibacter tataouinensis TTB310 to coordinate its cell cycle with water availability in dry, hot deserts. Genome analysis of Ramlibacter tataouinensis TTB310 also revealed the presence of genes relating to its unusual cell cycle, such as a very large variety of intra- and extracellular signal transduction channels.
This discovery enhances current knowledge of bacterial cell cycles, and, with regard to Ramlibacter tataouinensis TTB310, provides greater insight into how this cell cycle operates in connection with the molecular clock to enable the bacterium to adapt to desert conditions.
 Betaproteobacteria: one of five subclasses of proteobacteria (formerly referred to as "gram-negative bacteria").
 Desiccation: a dehydration process aimed at eliminating as much moisture as possible.
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