Snyder and Champness Molecular Genetics of Bacteria. Tina M. Henkin

Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin


Скачать книгу
href="#ulink_ca7a5e95-9e16-5cd6-96ae-247cd02d4f1b">Figure 1.14). The ability of DnaA to bind these high-affinity sites at all times can be considered analogous to the origin recognition complex associated with eukaryotes. Additional sites called “I” and “τ” sites, which differ from DnaA boxes, exist in the origin region but are occupied only by DnaA that is bound to ATP and not ADP (see below) (Figure 1.14). Finally, within an ATrich region of DNA that is opened for initiation, called the DNA-unwinding element, are three additional sites for DnaA binding that are occupied only when DnaA is bound to ATP and not ADP. Binding sites for other DNA-binding proteins (IHF and Fis) are also found in this region.

Schematic illustration of the structure of the origin of chromosomal replication region of Escherichia coli.

      INITIATION PROTEINS

      Besides the cis-acting oriC site and DnaA, many trans-acting proteins are also required for the initiation of DNA replication, including the DnaB and DnaC proteins. DnaA is required only for initiation, allowing DnaC to load the DnaB helicase for establishing the DNA replication forks. Many proteins used in other cellular functions are also involved, such as the primase (DnaG), the normal RNA polymerase that makes most of the RNA in the cell, and the DNA-binding proteins IHF and Fis (Figure 1.14).

      As described above, RNA primers are continuously needed during the DNA replication process. The complex that travels along the chromosome laying down RNA primers is called the primosome and contains DnaB and DnaG primase. The RNA polymerase that synthesizes most of the RNA molecules, including mRNA, in the cell (see chapter 2) is needed to initiate rounds of replication; however, transcription from RNA polymerase in this context from adjacent genes is involved in controlling the separation of the strands of DNA in the oriC region. DnaG primase is responsible for laying down RNA primers for DNA synthesis after replication is initiated at oriC.

Schematic illustration of the initiation of replication at the Escherichia coli origin region. DnaA is always bound to three DnaA boxes within oriC, even when DnaA is in its non-ATP-bound state acting as an origin recognition complex. Schematic illustration of the termination of chromosome replication in Escherichia coli. (A) The replication forks that start at the origin of chromosomal replication can traverse terA and terB in only one direction, opposite that indicated by the black arrows. (B) When they meet, between or at one of the two clusters, chromosome replication terminates.
Скачать книгу