Transformation in the cyanobacterium Synechococcus R2: improvement of efficiency; role of the pUH24 plasmid.

Systematic studies of the parameters influencing transformation efficiencies in Synechococcus R2 and its spontaneous pUH24 plasmid-minus variant S60, have led to substantial increases in these frequencies (up to 70 times for R2 and 9 times for S60) compared to those obtained by using previously published procedures. Important factors are the physiological state of the host cells (competence occurs at the transition from first to second exponential phases of growth) and the conditions of expression of the transferred characters (they must be exposed to progressively increasing concentrations of the selective agent). Consistently lower (about 30 times) efficiencies were obtained with the S60 strain, the only difference from the wild-type R2 that could be detected. Physiological functions of the pUH24 plasmid are discussed. The interest of the S60 strain as a host in gene cloning work is discussed in the light of published work suggesting inter-vector recombination in R2.