Small Proteins and Polyamines

Various reasons have been put forward as to why the spore is less sensitive to H202 than the vegetative cell. In the spore, the DNA is covered by the so-called small acid-soluble proteins (SASP) instead of water which makes it relatively inaccessible to oxidative damage as these SASPs may provide compaction besides sacrificial protection (e.g., Setlow 1994,1995 Setlow and Setlow 1993). 

Polyamines are ubiquitous cell components essential for normal growth and maybe targets for therapeutic intervention (Marton and Pegg 1995). The protecting effects of polyamines is even more pronounced than that of proteins, such as, for example, the bacterial MCI protein or the lac suppressor to their binding sites (Isabelle et al. 1993, 1999). Multiply protonated polyamines such 

This polyamine-induced compaction and aggregation of DNA (PICA effect Gos-ule and Schellman 1976) leads to a protection of DNA against OH-attack much beyond that expected on account of the OH-scavenging capacity of the polyamines (k( OH + putrescine) = 7 x 109 dm3 mol-1 s 1, k( OH + spermine) = 1.2 X 109 dm3 mol-1 s-1 Newton et al. 1996 Douki et al. 2000). Putrescine with only two positive charges cannot compact DNA and thus does not provide radiation protection beyond its OH scavenging capacity (Spotheim-Maurizot et al. 1995a Newton et al. 1996 Fig. 12.15). 

DNA compaction decreases the surface area per molecule exposed to bulk OH by 14%. Based on the cylindrical model (Mark et al. 1989), it has been calculated that competitive scavenging leads to a protection factor of 2, while a factor of 50 appears as a consequence of compaction (Newton et al. 1996). There is also a -20% contribution by the direct effect of ionizing radiation on DNA in this special system. 

Since in this section compaction of DNA by spermidine has been discussed, it may be worth mentioning that compaction by spermidine has also an effect on the photochemistry of DNA. While in non-compacted DNA besides the cis-syn pyrimidine dimer which requires B-form DNA the formation of some trans-syn pyrimidine dimer is always observed, but when the DNA is compacted with spermidine, the formation of the latter is practically fully suppressed (Douki et al. 2004). 

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The biomimetic studies of biomineralization have also provided inspiration to explain biopolymer-mediated silica formation in the diatoms. As mentioned in the previous section, biogenic silica contain small proteins and polyamines such as the silaffins. 

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