The RecJ DNase strongly suppresses genomic integration of short but not long foreign DNA fragments by homology-facilitated illegitimate recombination during transformation of Acinetobacter baylyi

Homology-facilitated illegitimate recombination (HFIR) promotes genomic integration of foreign DNA with a single segment homologous to the recipient genome by homologous recombination in the segment accompanied by illegitimate fusion of the heterologous sequence. During natural transformation of Acinetobacter baylyi HFIR occurs at about 0.01% of the frequency of fully homologous recombination. The role of the 5' single-strand-specific exonuclease RecJ in HFIR was investigated. Deletion of recJ increased HFIR frequency about 20-fold compared with wild type while homologous recombination was not affected. Illegitimate fusion sites were predominantly located within 360 nucleotides away from the homology whereas in wild type most fusion sites were distal (500-2500 nucleotides away). RecJ overproduction reduced the HFIR frequency to half compared with wild type, and transformants with short foreign DNA segments were diminished, leading to on average 866 foreign nucleotides integrated per event (682 in wild type, 115 in recJ). In recJ always the 3' ends of donor DNA were integrated at the homology whereas in wild type these were 3' or 5'. RecJ apparently suppresses HFIR by degrading 5' non-homologous DNA tails at the post-synaptic stage. We propose that the RecJ activity level controls the HFIR frequency during transformation and the amount of foreign DNA integrated per event.