Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 75

Proteins bound to, or otherwise acting on, the chromosome can also stop the progression of DNA polymerase III. A programmed block to DNA replication occurs in some bacteria to terminate DNA replication within one region of the chromosome (see below). However, unintended blocks can occur in other situations, such as when DNA polymerase encounters RNA polymerase carrying out transcription (see chapter 2) or when RNA polymerase is stalled at sites of damage in the chromosome. Transcription appears to be the most significant impediment to replication, but a variety of protein complexes need to be displaced ahead of the DNA replication fork.

In addition to DnaB, other helicases help DNA replication to proceed through obstacles on template DNAs. Experimentally, it was shown that two other E. coli helicases, Rep and UvrD, allow replication to proceed through a protein block on template DNA (see Guy et al., Suggested Reading). These helicases travel in the 3′-to-5′ direction, and therefore, they are likely to travel on the leading-strand template while DnaB progresses forward on the lagging-strand template (i.e., the 5′-to-3′ direction). Rep interacts directly with the DnaB helicase, probably as a normal part of the replisome. The replisome is the collection of proteins that interact with one another (through DNA or other proteins) and that are involved in carrying out DNA replication. In E. coli, UvrD may be a helicase of general use for helping out when DNA replication is blocked by proteins or to remove recombination structures from the chromosome to allow DNA replication to proceed. As explained in more detail in chapter 10, UvrD allows strand displacement during mismatch repair and plays additional roles in nucleotide excision repair in E. coli in a repair process that is coupled with transcription.