Introduction. Two types of prokaryotic cloning vectors can be distinguished: extrachromosomal plasmids, which cause antibiotic resistance of transformed cells, and bacteriophages like phage λ and filamentous phages, which are mostly selected by plaque formation. Among plasmids the most versatile vectors for transformation are those which are derived from the ColE1 plasmid, such as pBR322, pBR325 and pBR328 (Bolivar , 1977; Bolivar, 1978). They multiply to more than 50 copies per cell, and are relatively insensitive to the size of the insert. They do not autonomously transfer to other cells, although they can be mobilized by other plasmids like pRK2013 (Ditta , 1980). Improvements in cellular transformation methods (Dagert & Ehrlich, 1979; Hanahan, 1983) have increased the efficiency of uptake of DNA for some strains to more than 1 × 10 transformants per µg DNA.


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