In Escherichia coli, three isoforms of the essential translation initiation factor IF2 (IF2-1, IF2-2, and IF2-3) are generated from separate in-frame initiation codons in infB. The isoforms have earlier been suggested to act differentially in DNA replication restart. We report that in synthetic lethal situations associated with trapped Holliday junctions caused by deficiency of enzymes RuvAB or RuvC (that act in the post-synaptic step of homologous recombination [HR]), viability is restored in

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... bsence of any of the following: (i) IF2-1, (ii) RecA, which is the central protein for synapsis in HR, or (iii) proteins of the RecFORQ pre-synaptic HR pathway; conversely, loss of IF2-2 and IF2-3 exacerbated the synthetic defect. Strains lacking IF2-1 were also profoundly sensitive to two-ended DNA double-strand breaks (whose repair is mediated by RecA through the RecBCD pre-synaptic HR pathway), which was accompanied by reduction in extent of DNA loss around a break site. In HR assays, recovery of recombinants was diminished in IF2-1's absence. Our results suggest that isoforms IF2-1 and IF2-2/3 exert opposite effects at a step downstream of the two pre-synaptic pathways and of RecA nucleoprotein assembly, so as to increase and decrease, respectively, the efficiency of synapsis during HR