Microbial Genetics (Micr340)

Lecture 13Molecular Basis of

Recombination

Two Types of Recombination

Nonhomologous recombination Occurs very rarely Requires special proteins

Homologous recombination Occurs much more often Occurs between same or very similar

sequences

Recombination requirements

1. Identical or very similar sequences in the cross-over region

2. Complementary base pairing between double-stranded DNA molecules

3. Recombination enzymes4. Heteroduplex formation

Models of recombination Holliday double-strand invasion model

Migration of Holliday junctions

Single-strandInvasion

Model

Double-strand Break Repair Model

Molecular Basis for Recombination in E. coli

chi () Sites and the RecBCD nuclease Synapse Formation and the RecA

Protein The Ruv and RecG proteins and the

migration and cutting of Holliday Junctions

chi site and

RecBCD enzyme

Synapse formation

Model for action of

Ruv proteins

Model for action of Ruv proteins

(continued)

Model for action of

Ruv proteins

(continued)

Phage Recombination Pathways

Rec Proteins and Phages T4 and T7 Gene 49 protein of T4 and gene 3 protein of T7

are similar to RuvC Gene 46 and 47 products are similar to RecBCD

The RecE pathway of the rac prophage RecE protein can substitute for RecBCD

nuclease The phage red system

Red system requires genes exo and bet. Does not require RecA protein

Genetic Analysis of Recombination in Bacteria

Isolating Rec- mutants in E. coli Other recombination genes Gene conversion and other

manifestations of heteroduplex formation during recombination

Repair of a mismatch

Repair of a mismatch(continued

)

Outcome of a

mismatch repair