Goldilocks and RNA: where Mg2+ concentration is just right.

Magnesium, the most abundant divalent cation in cells, catalyzes RNA cleavage but also promotes RNA folding. Because folding can protect RNA from cleavage, we predicted a 'Goldilocks landscape', with local maximum in RNA lifetime at Mg2+ concentrations required for folding. Here, we use simulation and experiment to discover an innate and sophisticated mechanism of control of RNA lifetime. By simulation we characterized RNA Goldilocks landscapes and their dependence on cleavage and folding parameters. Experiments with yeast tRNAPhe and the Tetrahymena ribozyme P4-P6 domain show that structured RNAs can inhabit Goldilocks peaks. The Goldilocks peaks are tunable by differences in folded and unfolded cleavage rate constants, Mg2+ binding cooperativity, and Mg2+ affinity. Different folding and cleavage parameters produce Goldilocks landscapes with a variety of features. Goldilocks behavior allows ultrafine control of RNA chemical lifetime, whereas non-folding RNAs do not display Goldilocks peaks of protection. In sum, the effects of Mg2+ on RNA persistence are expected to be pleomorphic, both protecting and degrading RNA. In evolutionary context, Goldilocks behavior may have been a selectable trait of RNA in an early Earth environment containing Mg2+ and other metals.

Lanthanide rarity in natural waters: implications for microbial C1 metabolism.

Research in the last decade has illuminated the important role that lanthanides play in microbial carbon metabolism, particularly methylotrophy. Environmental omics studies have revealed that lanthoenzymes are dominant in some environments, and laboratory studies have shown that lanthoenzymes are favored over their calcium-containing counterparts even when calcium is far more abundant. Lanthanide elements are common in rocks but occur at exceedingly low levels in most natural waters (picomolar to nanomolar range) with the exception of volcanic hot springs, which can reach micromolar concentrations. Calcium is orders of magnitude higher in abundance than lanthanide elements across natural settings. Bacteria that use lanthanides for growth on simple carbon compounds (e.g. methanol and ethanol) grow optimally at micromolar concentrations. It is highly likely that bacteria in the environment have evolved specialized lanthanide sequestration and high-affinity uptake systems to overcome lanthanide deprivation. Indeed, we identified genes in soil metagenomes encoding the lanthanide-binding protein lanmodulin, which may be important for cellular differentiation between calcium and lanthanides. More research is needed on microbial adaptations to lanthanide scarcity.