Initiation of chromosomal DNA synthesis in vitro

Enzymatic replication of the origin of the Escherichia coli chromosome

An enzyme system that replicates plasmids bearing the origin of the Escherichia coli chromosomes (oriC) has the following physiologically relevant features. The system (i) depends completely on low levels of exogenously furnished supercoiled oriC plasmids, (ii) uses only those plasmids that contain the intact oriC region of about 245 base pairs, (iii) initiates replication within or near the oriC sequence and proceeds bidirectionally, (iv) proceeds linearly, after a 5-min lag, for 30-40 min to produce as much as a 40% increase over the input DNA, (v) depends on RNA polymerase and gyrase as indicated by total inhibition by rifampicin and nalidixate, (vi) depends on replication proteins (e.g., dnaB protein and single-stranded DNA binding protein) as judged by specific antibody inhibitions, (vii) operates independently from protein synthesis, and (viii) depends on dnaA activity, as suggested by the inactivity of enzyme fraction from each of two dnaA temperature-sensitive mutant strains, and complementation (with a 15-fold overproduction of complementing activity) by a fraction from a strain containing the dnaA gene cloned in a multicopy plasmid. Resolution and analysis of factors that control the initiation of a chromosome cycle should become accessible through its enzyme system.

RNA polymerase is required for DNA initiation in vitro

We have previously reported in vitro complementation assays for chromosome initiation that enable dnaA and dnaC mutant extracts to synthesize DNA. To examine the role of RNA polymerase in chromosome initiation, inhibitors of the enzyme and anti-RNA polymerase antibody were used. Though rifampicin failed to efficiently inhibit ribonucleoside triphosphate polymerization under the assay conditions, both streptolydigin and anti-RNA polymerase antibody abolished ribonucleic acid synthesis completely. Antibody effectively inhibited chromosome initiation in the dnaA mutant based reaction but streptolydigin did not. Neither streptolydigin nor antibody affected the dnaC-dependent assay. It was concluded that RNA polymerase is required for initiation but not necessarily to polymerize a polyribonucleotide. A scheme for the sequence of initiation events is presented.

Complementation of a dnaC initiation defect in vitro

The dnaC28 mutant, CT28-3b, is an initiation defective dnaC strain. Extracts of the mutant failed to synthesize DNA in vitro when the strain was incubated at the restrictive temperature for two generation times prior to preparation of the extract. Addition of a complementing extract from a Col-E1::dnaC+ hybrid plasmid containing strain or of partially purified dnaC protein resulted in substantial synthesis. Hybridization of the DNA made by these in vitro complementation extracts showed that a significant portion of this DNA was from the region near the chromosomal origin of replication.