Gin is the site-specific recombination system of bacteriophage mu. It mediates the inversion of the G-segment of the phage genome resulting in the expression of alternate sets of tail fiber genes whose products determine the host range of the phage. Gin is a highly specific recombinase that requires for its action the presence of three accessory factors: the protein Fis, an enhancer sequence that contains the binding sites for Fis, and negatively supercoiled DNA substrate. The tangle model of Sumners et al (QRB 28, 3 (1995), pp.253-313) is a mathematical tool which uses the topology of knotted and catenated DNA products of enzyme action to calculate changes in the topology of the DNA molecules. By doing tangle analysis of Gin data we proved that the enzyme found in nature uses a (-2,0) tangle as synaptic complex. Now, in order to better understand Gin's specificity to the (-2,0) synaptic complex and the roles of the accessory factors in recombination, a mutant of Gin that requires neither Fis nor negative supercoiling has been studied (Crisona et al, J Mol Biol 243,3 (1994),pp.437-457). I will use the tangle model to prove that mutant Gin must use a variety of different synaptic complexes and that these are all of the form (-n,0) for a positive integer n. The next questions that arise are: how many of these synaptic complexes are actually used? Given a knotted product of mutant Gin recombination, which are the possible pathways that lead to it? The answers to this questions would help to understand the mechanism of the mutant Gin and other recombinases, such as Int.
 
 
 

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