核酸感染系の特質
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概要
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Properties of various transfection systems in E. coli have been surveyed. The recipient preparations include various spheroplasts (induced by lysozyme, penicillin, or glycine), plasmolyzed or osmotically-shocked cells, cell-envelope mutants, helper-infected bacteria, frozen-thawed cells and bacteria treated with Ca<SUP>2+</SUP> or Ba<SUP>2+</SUP>. Efficiency of transfection depends on structure of nucleic acids as well as on competence of recipients. Thus, spheroplasts preferentially take up single-stranded nucleic acids, whereas Ca<SUP>2+</SUP>- or Ba<SUP>2+</SUP>-treated cells exhibit higher efficiency for double-stranded DNA. Unlike intact phages, free nucleic acids are sensitive to cellular nucleases even after uptake, and this sensitivity is also determined by structure of the infecting molecule (strandedness, circularity etc.). Mechanism of polynucleotide uptake into cytoplasm is quite obscure as yet. Although experimental evidence is lacking, it seems not improbable that nucleic acid penetrates a proteinous channel (or pore) rather than phospholipid layer. The Ca<SUP>2+</SUP>- or Ba<SUP>2+</SUP>-dependent uptake of DNA occurs very rapidly at low temperature and extremely sensitive to phosphate. Based on peculiar cryophilic property, an involvement of lipid crystallization has been postulated in this system. Such lipid crystallization might cause conformational change of protein pore to allow penetration of nucleic acid. Structure of the putative pore may be modulated by arrangement of lipopolysaccharides and other membrane proteins. Neutralization of negative charge on cell envelope is probably another effect of Ca<SUP>2+</SUP> or Ba<SUP>2+</SUP>.
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