SUMMARY: A strain of Rhizobium cowpea transformed interspecifically and intra-specifically for the gelatinase marker showed maximum competence in the early log phase of growth which was increased further by the addition of Casamino acids and Mg2+ to the competence medium at pH 7.0. Ferric ions inhibited transformation and this effect was partially reversed by addition of EDTA. The DNA concentration for maximum frequency of transformation was about 20 μg./ml. The frequency of intraspecific transformation was ten times higher than that of interspecific transformation although the G + C mole per cent of the donor and recipient was very similar, 64.0 and 64.5 respectively. A surface-bound nuclease specific for the transforming DNA could have resulted in the high donor DNA requirement.
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