Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 80

One cis-acting site involved in DNA replication is the oriC site, at which replication initiates. The sequence of oriC is well defined in E. coli, and the basic components that make up oriC are broadly similar in most bacteria. Figure 1.14 shows the structure of the origin of replication of E. coli. Less than 260 base pairs (bp) of DNA is required for initiation at this site. Within oriC are a series of binding sites for various proteins; the most important of these binding proteins is the master initiator protein in bacteria, called DnaA (see below). The canonical DnaAbinding sequences are 9 bp in length, and these sites are termed DnaA boxes. While five DnaA boxes exist within oriC, three of these sites bind DnaA particularly strongly, i.e., are of particularly high affinity, and are always bound by DnaA (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.

Figure 1.14 Structure of the origin of chromosomal replication (oriC) region of Escherichia coli. Shown are the positions of multiple types of DnaA-binding sequences, five DnaA boxes (R1 to R5) and other DnaA-binding sites (I and τ), and an AT-rich region that is unwound to allow loading of the replication apparatus, the DNA-unwinding element (DUE). Also shown are binding sites for the IHF and Fis proteins and a large number of GATC sites (black dots) that are important in regulating initiation by acting as sites of Dam methylation.