Abstract

In Escherichia coli, RecA protein catalyzes DNA pairing and strand exchange activities essential for genetic recombination. This is critical for normal cellular function under conditions that lead to altered DNA metabolism and DNA damage. The RecA proteins of E. coli and Bacillus halodurans both can bind to DNA and catalyze the specific proteolytic cleavage of LexA and lambda repressor which induces SOS response. At neutral pH self-cleavage of LexA depends exclusively on its binding to RecA filament, while at elevated pH (~10) it autodigests in the absence of RecA. We have shown in this work that the RecA-mediated cleavage and the binding of RecA to DNA promoted by B. halodurans are similar to those promoted by E. coli RecA, excepted that in the case of B. halodurans the rate of the cleavage reactions is increased at alkaline pH and that NaCl favors the binding of RecA to DNA. The results lead to two hypotheses for the pathway for RecA-mediated cleavage, in which we first suppose that the internal pH of the bacteria is neutral. Thus LexA cannot undergo autodigestion, the genes involving in DNA repair and replication are not transcribed, regulating the growth of the cell. The second hypothesis is that the external environment and the internal pH of the bacteria are alkaline; here also the bacteria may have developed strategies to maintain LexA not inactivated. These observations suggest that the LexA autodigestion in B. halodurans at high pH may be regulated at the transcriptional level and that B. halodurans may be haloalkaliphile bacterium.

Key words: DNA binding, haloalkaliphile, autodigestion.