Reckless DNA Degradation

Abstract

Escherichia coli recA mutants are devoid of RecA homologous recombination protein. They suffer from abnormal DNA degradation that starts at sites of double-strand DNA breaks (DSBs) and is carried out by RecBCD exonuclease. Since it causes an enormous loss of chromosomal DNA, it is termed the ‘reckless’ DNA degradation. The degradation is stimulated by DNA-damaging agents that introduce DSBs into the DNA. In wild-type E. coli cells, the RecBCD enzyme exhibits only moderate DNA degradation activity and plays an important role during recombinational repair of DSBs. Such constructive behavior of the RecBCD enzyme results from its interaction with specific octanucleotide sequence called Chi that is present throughout E. coli chromosome. The Chi sequence transforms RecBCD from a voracious exonuclease into a recombinase that produces 3′-terminated single-strand DNA (ssDNA) tail and directs loading of the RecA protein onto it. The ssDNA coated with RecA protein initiates recombination with homologous DNA duplex to promote DSB repair. In the absence of functional RecA protein, both Chi-dependent modification of RecBCD and DSB repair are abolished. The role of RecA in Chi-dependent RecBCD modification is not completely understood.