Microbial Genetics (Micr340) Lecture 13 Molecular 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 Identical or very similar sequences in the cross-over region Complementary base pairing between double-stranded DNA molecules Recombination enzymes Heteroduplex formation

Models of recombination Holliday double-strand invasion model

Migration of Holliday junctions

Single-strand Invasion 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 l red system Red system requires l 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