BACTERIAL ICE NUCLEATION - SIGNIFICANCE AND MOLECULAR-BASIS

Several bacterial species are able to catalyze ice formation at temper atures as warm as -2-degrees-C. These microorganisms efficiently catal yze ice formation at temperatures much higher than most organic or ino rganic substances. Because of their ubiquity on the surfaces of frost- sensitive plants, they are responsible for initiating ice formation, w hich results in frost injury. The high temperature of ice catalysis co nferred by bacterial ice nuclei makes them useful in ice nucleation-li mited processes such as artificial snow production, the freezing of so me food products, and possibly in future weather modification schemes. The rarity of other ice nuclei active at high subfreezing temperature , and the ease and sensitivity with which ice nuclei can be quantified , have made the use of a promoterless bacterial ice nucleation gene va luable as a reporter of transcription. Target genes to which this prom oter is fused can be used in cells in natural habitats. Warm-temperatu re ice nucleation sites have also been extensively studied at a molecu lar level. Nucleation sites active at high temperatures (above -5-degr ees-C) are probably composed of bacterial ice nucleation protein molec ules that form functionally aligned aggregates. Models of ice nucleati on proteins predict that they form a planar array of hydrogen binding groups that closely complement that of an ice crystal face. Moreover, interdigitation of these molecules may produce a large contiguous temp late for ice formation.