DNA repairing complex RecBCD
Escherichia coli (bacteria)
The maintenance of genetic stability, i.e. minimum variation in the DNA sequence, is essential for the correct functioning of cells and hence organisms. DNA is extremely liable to spontaneous changes due to exposure to various external stimuli such as heat, radiation, and other DNA damaging substances. These random changes in the DNA sequence need to be corrected or repaired efficiently. Accumulation of such changes or mutations results in their propagation during DNA replication, and in the production of dysfunctional or mutant proteins that are harmful to the cell. Indeed, the inability of cells to repair damaged DNA causes various diseases in humans such as cancer, neurological abnormalities and congenital abnormalities, among others. DNA damage can occur in various forms such as spontaneous changes in single bases, single strand breaks or double strand breaks. Each form of DNA damage is processed and repaired by different sets of enzymes that specifically recognize it. In eubacteria, double strand breaks are processed by a multi-enzyme complex, RecBCD, and repaired by homologous recombination. When DNA suffers double strand breaks, the RecBCD complex binds to the broken end of DNA and initiates the repair process. RecBCD moves along the DNA, and, as it separates and unwinds it, cleaving the single strands into smaller fragments. It stops cleaving the 3'-5' strand when it recognizes a specific sequence on the DNA called the Chi (Crossover hotspot instigator) site. It then recruits the protein RecA on the 3' tail to prevent further cleavage and damage to it. The DNA is then repaired by homologous recombination involving this 3' tail.
The structure shown here is that of the RecBCD complex in E. coli. It consists of 3 subunits - RecB, RecC and RecD. RecB is a 3'-5' helicase, having the ability to unwind single stranded DNA in the 3'-5' direction. It is also a nuclease i.e. it can cleave single stranded DNA into small fragments. Rec C splits the DNA into two single strands and recognizes the Chi site on the 3'-5' strand. It binds to the Chi site and prevents further cleavage of the strand. Rec D is a 5'-3' helicase. The DNA first enters the complex supported by a RecB arm. It is then split by RecC and the two single strands are fed to RecB and Rec D respectively, for unwinding. The strand that is unwound by RecB again enters RecC where it binds if it contains the Chi site. Both the strands are then fed to the RecB nuclease subunit for cleavage. Thus all the 3 subunits are tightly bound to each other with multiple interactions and function in concert with each other.
Protein Data Bank (PDB)
author: Ashwini Patil