Molecular Genetics

Facts

E. coli recA can bring two DNAs in close proximity for eventual recombination:

	The recA protein binds single-stranded DNA, incorporating the DNA into a helical complex with 6 recA molecules and 22 nucleotides/ turn. Adjacent bases are 1.5 times as far apart as they are in a B-DNA double helix. The complex is called a presynaptic filament.
RecA alone binds poorly to double-stranded DNA.
The presynaptic filament binds double-stranded DNA, non-specifically, causing large aggregates to form (ref). ATP is required for presynaptic filament formation and formation of the aggregates. Non-hydrolysable analogues can substitute for ATP.
	These aggregates are not stable to deproteination.
The aggregates contain paranemic joints. Paranemic is a topological term for the joint that is made by wrapping one circle around another without cutting either circle. The two circles can always be pulled apart. Paranemic is distinguished from plectonemic. Presynaptic filaments also bind to naked single-stranded DNA. This reaction is thought different from that involved in strand exchange.

Interpretations

• RecA monomers have two binding sites for DNA, one for single-stranded and one for double-stranded.
• The single-stranded DNA must be bound in the interior of the recA helix because its backbone is not long enough to wrap around the fiber.
• By binding double-stranded DNA, presynaptic filaments stimulate the search for identical sequences by limiting the search from three dimensions to one dimension.
• Since paranemic joints can be demonstrated in vitro, pairing of duplexes may precede breaks.

Further information

• Other organisms, including eucaryotes, have proteins similar in structure and function to recA (on-line review).
• RecA structure is related to its function.