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Title: Regulation of the endonuclease activity of type 1 restriction-modification systems
Author: Makovets, Svetlana
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1999
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Efficient acquisition of the genes (hsdR, M and S) that specify EcoKI and EcoAI, representatives of two families of type I restriction and modification (R-M) systems, was shown to require a product of an unknown gene hsdC. The hsdC mutant is shown to have a mutation in clpX. ClpP, the components of ClpXP protease, are necessary for the efficient transmission of the hsd genes by conjugation, transformation and P1 transduction. Inactivation of clpX leads to a bigger barrier than a similar mutation in clpP consistent with a chaperone activity of ClpX in the absence of ClpP. The establishment of the modification activity of EcoKI is not dependent on clpX and takes about 12 generations to reach its maximal activity in methylating incoming phage DNA. This lag probably reflects the time necessary to complete the methylation of bacterial chromosomes. Modification, once established, has been assumed to provide adequate protection against a resident restriction system. However, unmodified targets may be generated in the DNA of an hsd+ bacterium as the result of replication errors or recombinant-dependent repair. The presence of unmodified target sequences for type I restriction-modification systems on bacterial chromosomes does not influence the survival of hsd+ bacteria due to ClpXP- dependent regulation of the endonuclease activity. HsdR, the polypeptide of the R-M complex essential for restriction but not modification, is degraded in the presence of ClpXP and therefore the bacteria show a temporary drop in restriction activity, referred to as restriction alleviation. The delayed detection of restriction activity followed by the establishment of a new specificity can be considered as a case of restriction alleviation. The data obtained support a model in which the HsdR component of a type I restriction endonuclease becomes a substrate for proteolysis after the endonuclease has bound to unmodified target sequences on the chromosome, but before completion of the pathway that would result in DNA breakage. It remains unclear how the restriction-modification systems distinguish between unmethylated host and foreign DNA. The latter is degraded while the former is protected from cleavage by ClpXP-dependent proteolysis of HsdR.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available