Question

15. Briefly describe the structure that RecA/Rad51 create on ssDNA and what the primary functional purpose for this physical organization is in promoting strand invasion for homologous recombination. [5 points

Answer 1

Answer: RecA protein is a important one in bacterial homologous recombination, it is also called single exchange protein. Rec51 is eukaryotic protein homologous to bacterial RecA protein, in involve in homologous recombination, There are three combination system in bacteria- RecBCD System, RecE and Rec F system, among these RecBCD system is most important system in bacteria.

RecBCD System:

By this system recombination is started by the RecBCD enzyme, also designated as exonuclease v, This enzyme composed of 3 different subunits, termed as RecB, RecC, RecD. This enzyme complex act as a bipolar DNA helicase. RecC subunit identify specific sequence in DNA called as chi (5' GCTGGTGG3'). RecB and RecD both are ATP dependent helicase. RecB travels on one strand on DNA in 3'-5' direction, while RecD travels on DNA in 5'-3' direction. RecBCD enzyme binds to the linear DNA at broken site and moves inward along the double helix and open up the double helix. RecB is slower than RecD protein, leads to a single strand loop accumulation in front of RecB. When RecBCD enzyme reach the chi site activity of the enzyme alter. RecC identify chi site in in single stranded form. RecC signal the RecD to stop unwinding the DNA. RecD hen signal the RecB to cut the DNA. rECb cut the strand at chi site. Unwinding continues and FORMATION OF 3' single strand tail. RecA protein is then energetically  loaded by RecBCD enzyme onto the 3' tail. The protein RecA produces a protein covered DNA fragment which is able to invade double strand and can form the D loop and allow the tail. Search for homologous region, once homologous region recognised a nick occur in D loop. RecA protein augment the homologous DNA pairing. Once RecA protein loaded onto the DNA molecule it is become active. RecA protein binds more rapidly in single stranded DNA than double stranded DNA that is why single stranded DNA is for single stranded DNA in strand exchange substrate. RecA protein binds in 5'-3' direction, that gives rise to formation of 3' recA coated segment.