ý.,,: V. ý ýý . - Nottingham eTheses - University of Nottingham

(INA+ phenotype) bacteria are the most temperature-effective natural ice nuclei after ice
itself (which nucleates at 0°C under standard conditions (Yankofsky et al., 1997)). There
are a large number of other naturally occurring ice nucleators including detritus (Upper &
Vali, 1995), spider silk (Murase et al., 2001) and soot particles (Gorbunov et al., 2001).
The INA+ phenotype has been identified in three Gram negative genera: Erwina,
Pseudomonas and Xanthomonas. The protein itself is homologous between the genera,
forming a beta-helical structure which could interact with water through the repetitive
TXT motif (Graether & Jia, 2001). Water molecules are arranged into an ice-like array as
the membrane bound heteronucleator (INP) acts as a template that mimics the structure of
ice. INA+ bacteria are generally epiphytic and are often ubiquitous in agriculturally
important crops. They play a significant role in causing frost damage by promoting ice
formation at a higher temperature than would occur if normal super cooling temperatures
(-4°C to-12°C) were allowed to occur, accounting for an excess of $1 billion in crop
injury in the United States per year (Edwards et al., 1994). As a result of this, much
research has been performed to elucidate the mechanism behind ice nucleation in an
effort to control or prevent crop damage by these bacteria. Research into crop protection
led to the marketing of a number of commercial products including those used in the
prevention of frost (FrostbanTM), manufacture of snow (Snomax®), reduction of the
incidence of fire blight (Blightban®) and as an aid for food concentration and texturing
(Skirvin et al., 2000). For a comprehensive review of the history of ice nucleation and
nucleation function please refer to Skirvin et al. (2000).
1. S. 3.4 -
Antifreeze proteins
1.5.3.4.1- AFP structure and function
Ice-nucleating proteins induce ice formation but anti-freeze proteins inhibit ice
formation, and in some cases have been shown to inhibit the action of INPs (Parody-
Morreale et al., 1988). Anti-freeze proteins (also known as thermal hysteresis proteins)
are a structurally diverse class of proteins, which all have the ability to bind to the surface
of ice and inhibit its recrystalisation (Yeh & Feeney, 1996; Davies & Sykes, 1997;
Smaglik, 1998). They act at the surface of ice by lowering the freezing point below the
melting point (Davies & Sykes, 1997; Knight & DeVries, 1994; Haymet et al., 1998), a
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