Molecular Genetics

Regulation of DNA replication in E.coli differs from regulation in nucleated cells.

Facts

The dnaA gene product is required for the intitation of DNA replication. dnaA and ori (the origin of bidirectional replication of the E. coli chromosome) are close to one another on the genetic map. ori has 11 GATC sites, targets for dam methylation. Hemimethylated ori sequences and the dnaA gene are bound to a membrane protein. Unmethylated or fully methylated sequences are not. Addition of the dam methylase to membrane-ori dnaA complexes leads to release of the complexes. The release is dependent on the addition of the methyl donor substrate, SAM. When plasmids containing fully methylated ori are transferred into E. coli, their replication is delayed by 30 to 40% of a cycle. Dam- strains of E. coli have poor control of ori synchrony. The origins fire more randomly.

Interpretations

A model consistent with results:

• When replication creates hemimethylated DNA, the ori and dnaA genes are sequestered. In the sequestered state they are inactive in replication and transcription.
• The level of dnaA protein decays rapidly, so that from one cycle to the next, there is not enough old dnaA to start replication.
• The dam methylase eventually methylates all sites and results in the release of ori and dnaA.
• The released dnaA gene is transcribed and translated.
• The dnaA protein initiates DNA replication, creating new replication forks.

Further information

• The cycle of hemimethylation and full methylation can be used to follow the timing of replication (ref).
• Replication of bacterial plasmids is regulated by still different mechanisms.

This is page 1336 of Molecular Genetics by Ulrich Melcher, © 1998, 1999