Molecular aspects of microbial ice nucleation.
Certain organisms nucleate the crystallization of ice. This requires a small volume of water to be induced, probably by lattice-matching with a solid template, to form an 'ice embryo'--a region sharing at least some of the characteristics of macroscopic ice. It is of particular interest to understand the structure and function of biological structures capable of lattice-matching (or otherwise inducing a quasi-crystalline state). Some strains of the Gram-negative eubacterial genera Erwinia, Pseudomonas, and Xanthomonas, and the mycobionts of certain lichens, display ice-nucleating activity. In bacteria, the activity is conferred by a protein that contains three nested periodicities of repetition, which probably reflects a hierarchy of three motifs of structural repetition. Thus the tertiary structure of the ice-nucleation protein is likely to be regular, consistent with the expectation of its forming a template for lattice-matching. Even within a clonal culture, the nucleating sites formed by bacteria and lichens vary considerably in the threshold temperatures at which they display activity; this indicates wide variations in either the size of the template, or its structural regularity, or both. However, ice-nucleating sites of lichen and bacterial origin are clearly differentiated by their sensitivities to experimental treatments.