Two membrane sites for DNA synthesis in Pneumococcus

DNA replication by a DNA-membrane complex extracted from Bacillus subtilis: site of initiation in vitro and initiation potential of subcomplexes

A DNA-membrane complex extracted from Bacillus subtilis was studied further as a model system for initiation of bacterial DNA replication in vitro. Of three subcomplexes purified from the crude complex by a combination of CsCl and sucrose gradient centrifugation, the synthetic capability of only one was inhibited significantly by streptovaricin, a known inhibitor of RNA primer formation. A selective enrichment in the level of this subcomplex was obtained by manipulating a thymine-requiring mutant. The synthetic capabilities of an enriched and nonenriched DNA-membrane complex were compared in the presence and absence of streptovaricin. Although the rate and extent of DNA synthesis per unit of protein were approximately the same in the absence of the antibiotic, there was a much greater inhibition of synthesis shown by the enriched complex in the presence of streptovaricin. Although the amount of DNA present in the putative initiation subcomplex was less than 0.3 to 0.4% of the total DNA present in the crude complex, such DNA, except for a few quantitative differences, was still representative of genomic DNA. Newly synthesized DNA hybridized to specific origin- and non-origin-derived restriction fragments of the B. subtilis genome. However, when an elongation inhibitor (ddCTP) was added, hybridization of such DNA to almost all of the nonorigin fragments disappeared or was reduced drastically, whereas origin region hybridization patterns remained strong. The highest level of hybridization in the origin region occurred with a BamHI (B7) restriction fragment of 5.6 kilobases that has been implicated by others as one site initiation in vivo (N. Ogasawara, M. Seiki, and H. Yoshikawa, Nature (London) 281:702-704, 1979; S. J. Seror-Laurent and G. Henckes, Proc. Natl. Acad. Sci. USA 82:3586-3590, 1985).

Enrichment of DNA polymerase III activity in a DNA membrane complex purified from Pneumococcus: the possible existence of subcomplexes

Three DNA polymerase activities, one related to DNA pol III, have been extracted from a DNA membrane complex purified from Streptococcus pneumoniae. DNA pol III was purified 3300-fold, DNA pol II 2800-fold and DNA pol I 1800-fold. Based on inhibition analysis with a drug known to inhibit DNA pol III activity in Gram positive organisms. 6(p-hydroxyphenyl azo) uracil (HpU), 55% of the total DNA polymerase activity is represented by pol III. In contrast, only 3-5% of the total DNA polymerase activity is inhibited by HpU in crude extracts. The purification of the DNA membrane complex from pneumococcus is modified from an earlier procedure (Firshein 1972). The modified procedure results in the separation of three distinct DNA-protein-phospholipid subcomplexes of which the one described above contains most of the radioactivity derived from cells pulsed for a short time with (3H)-thymidine. Proteins are involved in binding DNA in each complex and the conformation of DNA in each complex may be different. All of the subcomplexes contain DNA polymerase activity partially sensitive to HpU. These results provide direct evidence for the structural integrity of a complex that may be involved in DNA replication in vivo.

Association of the folded chromosome with the cell envelope of Escherichia coli: nature of the membrane-associated DNA

Membrane-associated folded chromosomes isolated from Escherichia coli in the presence of spermidine sedimented at about 5,800S. The folded chromosome and the membrane fragment were each stable in the absence of the other; a 1,700S folded chromosome was obtained after removal of the membrane by a Sarkosyl treatment, and a 4,000S membrane fragment remained after digestion of the chromosomal DNA with deoxyribonuclease I. The interaction between the folded chromosome and the membrane fragment was stable, and, even when the DNA was unfolded, both components remained associated and cosedimented. The large frictional effect of the unfolded DNA reduced the sedimentation rate of the complex to about 2,000S. Partial removal of this unfolded DNA with restriction endonucleases caused the membrane fragments and the remaining associated DNA to sediment faster, at about 3,500S. The DNA remaining associated with the membrane fragments after restriction endonuclease treatment, about 4.5% of the total DNA when EcoRI was used, was indistinguishable from the DNA released from the membranes by three criteria: (i) DNA size distribution in agarose gels after electrophoresis, (ii) reassociation kinetics, and (iii) thermal elution from hydroxylapatite. This finding, that random DNA sequences rather than specific ones were responsible for the majority of the DNA-membrane interactions, argues against the folded chromosome's being a static structure with specific DNA sequences interacting with the cell envelope.