FIG. 3.

Copy number and intracellular ssDNA accumulation of pLS1 and its derivatives in S. pneumoniae. Total DNA was isolated from the indicated plasmid-containing cultures, electrophoresed on 0.7% agarose gels in the presence of ethidium bromide (A), and transferred to nitrocellulose with (B) or without (C) prior denaturation. Note that in the latter case, only ssDNA is visible. The different plasmid forms were detected by using radiolabeled pLS1 as a probe. Both autoradiograms were overexposed to reveal the weak bands corresponding to ssDNA from plasmids with mutations in the CS-6 region. The positions of the supercoiled monomeric plasmid form (ccc), open circular dsDNA (oc) and ssDNA are indicated. chr, chromosomal DNA.