Extremophile enzyme optimization for low temperature and high salinity are fundamentally incompatible

Genomic analyses reveal a low-temperature adapted clade in Halorubrum, a widespread haloarchaeon across global hypersaline environments

BACKGROUND

Cold-adapted archaea have diverse ecological roles in a wide range of low-temperature environments. Improving our knowledge of the genomic features that enable psychrophiles to grow in cold environments helps us to understand their adaptive responses. However, samples from typical cold regions such as the remote Arctic and Antarctic are rare, and the limited number of high-quality genomes available leaves us with little data on genomic traits that are statistically associated with cold environmental conditions.

RESULTS

In this study, we examined the haloarchaeal genus Halorubrum and defined a new clade that represents six isolates from polar and deep earth environments ('PD group' hereafter). The genomic G + C content and amino acid composition of this group distinguishes it from other Halorubrum and the trends are consistent with the established genomic optimization of psychrophiles. The cold adaptation of the PD group was further supported by observations of increased flexibility of proteins encoded across the genome and the findings of a growth test.

CONCLUSIONS

The PD group Halorubrum exhibited denser genome packing, which confers higher metabolic potential with constant genome size, relative to the reference group, resulting in significant differences in carbon, nitrogen and sulfur metabolic patterns. The most marked feature was the enrichment of genes involved in sulfur cycling, especially the production of sulfite from organic sulfur-containing compounds. Our study provides an updated view of the genomic traits and metabolic potential of Halorubrum and expands the range of sources of cold-adapted haloarchaea.

Diffusive intracellular interactions: On the role of protein net charge and functional adaptation

A striking feature of nucleic acids and lipid membranes is that they all carry net negative charge and so is true for the majority of intracellular proteins. It is suggested that the role of this negative charge is to assure a basal intermolecular repulsion that keeps the cytosolic content suitably 'fluid' for function. We focus in this review on the experimental, theoretical and genetic findings which serve to underpin this idea and the new questions they raise. Unlike the situation in test tubes, any functional protein-protein interaction in the cytosol is subject to competition from the densely crowded background, i.e. surrounding stickiness. At the nonspecific limit of this stickiness is the 'random' protein-protein association, maintaining profuse populations of transient and constantly interconverting complexes at physiological protein concentrations. The phenomenon is readily quantified in studies of the protein rotational diffusion, showing that the more net negatively charged a protein is the less it is retarded by clustering. It is further evident that this dynamic protein-protein interplay is under evolutionary control and finely tuned across organisms to maintain optimal physicochemical conditions for the cellular processes. The emerging picture is then that specific cellular function relies on close competition between numerous weak and strong interactions, and where all parts of the protein surfaces are involved. The outstanding challenge is now to decipher the very basics of this many-body system: how the detailed patterns of charged, polar and hydrophobic side chains not only control protein-protein interactions at close- and long-range but also the collective properties of the cellular interior as a whole.

Some Clues about Enzymes from Psychrophilic Microorganisms

Enzymes purified from psychrophilic microorganisms prove to be efficient catalysts at low temperatures and possess a great potential for biotechnological applications. The low-temperature catalytic activity has to come from specific structural fluctuations involving the active site region, however, the relationship between protein conformational stability and enzymatic activity is subtle. We provide a survey of the thermodynamic stability of globular proteins and their rationalization grounded in a theoretical approach devised by one of us. Furthermore, we provide a link between marginal conformational stability and protein flexibility grounded in the harmonic approximation of the vibrational degrees of freedom, emphasizing the occurrence of long-wavelength and excited vibrations in all globular proteins. Finally, we offer a close view of three enzymes: chloride-dependent α-amylase, citrate synthase, and β-galactosidase.