Transfection of Rhizobium meliloti with DNA isolated from phage 16-3 was achieved by infecting competent cells or spheroplasts. Because of the low transfection frequency a transfection system with helper phage was established. Two functions of the helper phage were revealed: (a) induction of DNA uptake; and (b) marker rescue of the transfectious DNA. The formation of transfectants is governed by the helper phage DNA, and the function of early genes of the helper phage is essential in this process. Marker rescue recombination was used to measure linkage between phage markers in transfection. The sequence of 6 temperaturesensitive markers determined in this way fitted well with the genetic map of the phage 16-3 derived from crosses.
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