Genetic and biochemical analysis of the synaptic complex of invertase Gin.

The Gin inversion system of bacteriophage Mu requires the formation of a synaptic complex of unique topology, where the two Gin dimers bound at the recombination gix sites are interacting to form an enzymatically active tetramer, which then catalyses the site-specific recombination reaction. After the assembly of the synaptic complex the DNA strand cleavage is activated by the DNA-bending protein FIS bound at the recombinational enhancer sequence. During reaction the complex undergoes conformational changes resulting in a site-specific inversion of a DNA segment in the phage Mu genome. In this thesis the protein interactions in the synaptic complex were analysed. First, the question on the interactions between FIS and Gin during formation of the synaptic complex was addressed. In a genetic test system a mutant fisS14P has been selected that can rescue the recombination-deficient phenotype of the mutant Gin H106T. FIS S14P was shown to activate the Gin H106T mutant in vivo but not in vitro. The possible reasons are the differences in the in vivo and in vitro conditions, and the observed altered DNA bending ability of the FIS S14P mutant. The position of the mutation S14P in the “β-hairpin arm” of the FIS N-terminus suggests it could directly interact with the hydrophobic dimerisation interface of Gin around the position H106. Next, the predictions of the preliminary model of the Gin invertasome organisation have been verified and the catalytic domains of Gin were demonstrated indeed to be involved in tetramer formation. To do this, specific mutations at the proposed synaptic interfaces were introduced and biochemical studies of different mutants of Gin invertase affected in their ability to promote synapsis were performed. It was possible to show that in addition to the already identified surfaces of the Gin dimer-dimer interactions, comprising of the αE helix and the flexible loop between the β2 sheet and the αB helix of Gin, also the αD helix and the loop between αA helix and β2 sheet are involved in the stabilisation of the Gin tetramer. Cysteine substitutions placed on these surfaces could be efficiently cross-linked in the tetramer in the presence of DNA and FIS, indicating their close proximity in the synapse. Furthermore, Gin mutants with either increased or decreased tetramerisation abilities were isolated and characterised, and the effects of these mutations on recombination were studied. These data led to the notion that the tetramer structure should be flexible, since all mutations that stabilise the complex cause inversion deficiency. In turn, the complexes formed by the hyperactive mutants seem to have high conformational flexibility, although at the expense of the loss of specificity. Notably, introduction of substitutions that stabilise the Gin tetramer also lead to suppression of hyperactive features. A chimeric recombinase protein, containing the N-terminal catalytic domain from Gin and the DNA-binding domain of ISXc5 resolvase, was found to form a more stable tetramer complex, than Gin. The chimera ISXc5G10 is inversion deficient, but can still catalyse resolution. Again, these observations support the notion that the stabilisation of the tetramer can strongly impair the ability to catalyse inversion, but may have less effect on the resolution activity. The DNA-binding domain of ISXc5G10 chimera was mutagenised to obtain a protein with an inversion proficient phenotype, but no mutants of this type could be found, perhaps because in the chimera not only the DNA binding domain, but the gross organisation of the protein is different. Thus, according to the obtained data the Gin dimers bound to the recombination sites are interacting with each other via catalytic domains and recombination involves gross reorganisations of contact surfaces. The obtained results allowed to clearly distinguish between the two previously proposed mechanistically different models of recombination (the “subunit exchange” and “static subunits” models), and favour the “subunit exchange” model. Such a model serves as a useful working hypothesis for future experiments dedicated to the detailed understanding of the mechanism of recombination reaction catalysis by members of the serine recombinase family.