MuGam homologue proteins - an analysis of function and viability as contributors to DNA repair

Until recently it was thought that the only accurate DNA double strand break (DSB) repair mechanism used by bacteria was homologous recombination (HR), recent discoveries have found proteins that are orthologous to eukaryotic Ku heterodimeric proteins. These prokaryotic Ku proteins like their eukaryotic counterparts function by biding non-specifically to the ends of DSBs to recruit a DNA ligase to join the two ends of the DNA back together, this mechanism is known as nonhomologous end joining (NHEJ). Bacteria that can utilize NHEJ to repair DNA DSBs are at a significant advantage to those which cannot but only a select few species contain a gene for the prokaryotic Ku protein. The protein Gam from bacteriophage Mu (MuGam) has been identified as possessing sequence homology with the eukaryotic Ku heterodimer which has led to research into the potential for this MuGam protein to contribute to the NHEJ repair mechanism as a functional orthologue to the eukaryotic Ku70/80 complex. One study found that introducing MuGam to Escherichia coli K-12 MG1655 cells lead to an increase in both survival rate and instances of accurate and inaccurate repair when the cells were subjected to a DSB inducing element. The gene for MuGam or similar slightly mutated genes (MuGam homologues) are commonly found in many different of bacteria. Here eight different MuGam homologues are shown to be capable of being overproduced in Escherichia coli K-12 MG1655 that are functional in binding linear DNA using electromobility shift assays (EMSA). The sequence similarity between MuGam and the eukaryotic Ku proteins is very low and secondary structure predictors do not support structural similarities between the eukaryotic Ku protein and the MuGam protein.