Sánchez-Romero, MA, Molina, F and Jiménez-Sánchez, A (2010) Correlation between ribonucleoside-diphosphate reductase and three replication proteins in Escherichia coli. BMC Mol. Biol. 11:11
There has long been evidence supporting the idea that RNR and the dNTP-synthesizing complex must be closely linked to the replication complex or replisome. We contributed to this body of evidence in proposing the hypothesis of the replication hyperstructure. A recently published work called this postulate into question, reporting that NrdB is evenly distributed throughout the cytoplasm. Consequently we were interested in the localization of RNR protein and its relationship with other replication proteins.
Bacterial Outer Membrane Proteins/metabolism; Bacterial Proteins/metabolism; Chromosomes, Bacterial; DNA Polymerase III/metabolism; DNA Replication; DNA-Binding Proteins/metabolism; DnaB Helicases/metabolism; Escherichia coli/enzymology; Escherichia coli Proteins/metabolism; Microscopy, Fluorescence; Protein Subunits/metabolism; Ribonucleoside Diphosphate Reductase/metabolism
This paper shows that NrdB, and consequently Ribonucleosidediphosphate reductase (RNR), is not evenly distributed throughout the E. coli bacterial cell but it is located in clusters that can be observed as discrete foci by immunofluorescence microscopy.
NrdB foci have the following properties:
1. All foci were always seen within the nucleoid.
2. NrdB foci are located very close to three replication proteins: DnaX or DNA polymerase III τ subunit, helicase DnaB, and SeqA protein.
3. Number of NrdB foci per cell is the same as the number of foci of the three replisome proteins (tested at three different cell cycle periods).
4. NrdB foci appear only in replicating cells and its number decreases as replications end.
The conclusion is that RNR must be associated with the replication complex or replisome. This association would predict that other proteins from the Mathew’s dNTP-synthesizing complex (4) could also be related to the replication fork. This association would require a higher order structure, the hyperstructure (19, 20), to assemble the many functionalities required for precursors biosynthesis, chromosome replication and segregation together with the required membrane structure.
Replication hyperstructure may have unique properties. One of them could be the formation of a discrete pouch that will allow compartmentation of replication precursors by reduction of their diffusion which will permit the precise allosteric regulation of RNR activity, and to attain the high concentrations of the dNTP required for a replicating fork demanding about 1,500 nucleotides per second. Moreover, the tight association among the several functionalities required for precursor biosynthesis, chromosome replication and segregation, and most likely cell division (18), replication hiperstructure would generate the required interconnections that will allow the tight control mechanisms that regulate bacterial cell cycle.
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