Properties and transforming activities of two plasmids in Streptococcus pneumoniae

Summary Two plasmids from group B streptococcus were introduced into pneumococcus (Streptococcus pneumoniae) and examined for copy number, stability, and some features of the process by which they transform pneumococcal recipients. The 3.6 Mdal pMV158 (tet) was present at a minimum of 12 to 16 copies per chromosome and was never observed to be cured. The 20 Mdal pIP501 (cat erm) had a minimum copy number of 3 to 4 per chromosome and was lost spontaneously at a frequency near 0.03 per division. The presence of novobiocin increased this frequency 2 to 3-fold. Competence for chromosomal transformation and the membrane endonuclease needed for normal DNA entry were required for plasmid transformation. Plasmid transformants segregated transformed cells one generation ahead of chromosomal transformants. Both single and multiple hit components of the transformation reaction kinetics were observed, but the latter could not be seen in the presence of competing chromosomal DNA. The majority of the transforming activity behaved as covalently closed circular DNA in dye-buoyancy gradients. Although most of the activity for both plasmids sedimented in sucrose gradients more rapidly than did monomeric closed circular DNA, a significant fraction was found at a position suggesting that it may have been due to monomeric plasmids.