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 Escherichia coli transformation are those which are derived from the ColE1 plasmid, such as pBR322, pBR325 and pBR328 (Bolivar et al., 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 E. coli cells, although they can be mobilized by other plasmids like pRK2013 (Ditta et al., 1980). Improvements in cellular transformation methods (Dagert & Ehrlich, 1979; Hanahan, 1983) have increased the efficiency of uptake of DNA for some E. coli strains to more than 1 × 108 transformants per µg DNA.