Summary: Conditions have been investigated which allow the mapping of the replication order of the genes on the pneumococcal chromosome by the density-shift method. Adequate synchrony of DNA replication was achieved by using cultures already involved in the strict regime designed to give competent cells; such cultures were grown for 30 min into the stationary phase and then diluted into the medium in which the density-shift will occur. The shift was achieved by changing from either (i) ‘light’ peptone medium to synthetic medium containing 5-BU, or (ii) peptone medium prepared in DO to ‘light’ peptone medium. Samples of such cultures were lysed and centrifuged in caesium chloride and the gradients so formed fractionated and analysed for the transforming activity for the genetic markers. A map giving the order of replication of eleven markers is described.


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