This document discusses different types of genetic recombination enzymes called recombinases. It describes two main types of recombination - homologous recombination, which involves exchange between similar DNA sequences, and site-specific recombination, which recombines DNA at specific sites regardless of homology. The key recombinases involved in homologous recombination are RecBCD and RecA in E. coli. There are two families of site-specific recombinases - serine recombinases like Hin that cleave all DNA strands, and tyrosine recombinases like Cre, Lambda and Xer that cleave strands one pair at a time.

1. CONTENTS
 Introduction
 Homologous & Site-specific recombination
 Recombinases involved in homologous
recombination
• RecBCD
• RecA
 Recombinases involved in Site-specific
recombination
 Types of site-specific recombinases
• Serine recombinases
- Hin recombinase
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2. • Tyrosine recombinases
- Cre recombinase
- Lambda recombinase
- Xer recombinase
 Conclusion
 References
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3. INTRODUCTION
 Recombinases are genetic recombination enzymes .
 An enzyme that catalyzes the exchange of short pieces
of DNA between two long DNA strands , particularly the
exchange of homologous regions between the paired
maternal & paternal chromosomes .
 These enzymes derived from bacteria & fungi ,
catalyze directionally sensitive DNA exchange
reactions between short (30 – 40 nucleotides) target
site sequences that are specific to each recombinase .
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4.  The recombinases bring these specific site
together to form a protein – DNA complex
bridging the DNA sites known as the synatic
complex .
 Within synaptic complex , the recombinase
catalyzes the clevage & rejoining of the DNA
molecules either to invert a DNA segment or to
move a segment to a new site .
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5. HOMOLOGOUS & SITE – SPECIFIC
RECOMBINATION
 Homologous recombination is the process where by
DNA segments that are identical or similar to each
other break & rejoin to form a new recombination .
 It involves an exchange between DNA segments that
are similar or identical in their DNA sequences .
 During site – specific recombination , non-homologous
DNA segments are recombined at specific sites .
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6. RCOMBINASES INVOLVED IN HOMOLOGOUS
RECOMBINATION
 Homologous recombination is found
in all species , & requires the
participation of many proteins & the
types of proteins that participate in the
steps are very similar .
 This process is best understood in
Escherichia coli .
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7. RECBCD
 The term Rec indicates that these proteins are involved
with recombination .
 RecBCD is a protein complex composed of the RecB ,
RecC & RecD proteins .
 The RecBCD complex plays an important role in the
initiation of recombination involving double-strand
breaks .
 In this process , RecBCD recognizes a double-strand
break within DNA & catalyzes DNAunwinding & strand
degradation .
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8.  The action of RecBCD produces single-stranded
DNA ends that can participate in strand invasion &
exchange .
 The single-stranded DNA ends are coated with
single-stranded binding protein to prevent their
further degradation .
 The RecBCD complex can also create breaks in
the DNA at sites known as chi-sequences .
 In E.coli , the chi sequence is 5’-GCTGGTGG-3’ .
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10. RECA
 The function of the RecA protein is to promote strand
invasion . To accomplish this task , it bins to the
single-stranded ends of DNA molecules generated
from the activity of RecBCD .
 A large number of RecA proteins bind to single-
stranded DNA , forming a structure called a filament .
 During strand invasion , this filament makes contact
with the unbroken chromosome .
 Initially , this contact is most likely to occur at non-
homologous regions .
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11.  The contact point slides along the DNA until it
reaches a homologous regions .
 Once a homologous site is located , RecA catalyzes
to displacement of one DNA strand & the invading
single-stranded DNA quickly forms a double helix
with the other strand This results in a D-loop .
 RecA proteins mediate the movement of invading
strand & the displacement of the complementory
strand . This occurs in such a way that the displaced
strand invades the vacant region of the broken
chromosome .
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12. RCOMBINASES INVOLVED IN SITE-SPECIFIC
RECOMBINATION
 Conservative site-specific recombination
(CSSR) is a recombination between two
defined sequence elements .
 Site-specific recombinases cleave& rejoin
DNA using a covalent Protein-DNA
intermediate .
 It is this mechanistic feature that contributes
the “conservative” to the CSSR name :- it is
called “conservative” because every DNA
bond is resealed by the recombinase .
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13. TYPES OF SITE-SPECIFIC RECOMBINASES
 There are two families of conservative site-specific
recombinase :- the serine recombinases & the tyrosine
recombinases .
 For the serine recombinase , the side chain of a serine
residue within the protein’s active site attacks a specific
phosphodiester bond in the recombination site .
 This reaction introduces a single stranded break in the
DNA & simultaneously generates a covalent linkage
between the serine & a phosphate at this DNA
cleavage site .
 Likewise , for the tyrosine recombinases , it is the side
chain of the active-site tyrosine that attacks & then
becomes joined to the DNA .
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15. SERINE RECOMBINASES
 Serine recombinases introduce Double-strand
breaks in DNA & then swap strands to
promote recombination .
 The serine recombinases cleave all four
strands prior to strand exchange .
 One molecule of the recombinase protein
promotes each of these cleavage reactions ,
therefore a minimum of four subunits (i-e a
tetramer) of the recombinase is required .
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17. HIN RECOMBINASE
 Hin belongs to the serine recombinase family
of DNA invertase & is a 21KD protein
composed of 198 amino acids that is found in
the bacteria Salmonella .
 The Salmonella Hin recombinase inverts a
segment of the bacterial chromosome to allow
expression of two alternative sets of genes .
 The chromosomal region inverted by Hin is
about 1000 bp & is flanked by specific
recombination sites called hix L (on the left) &
hix R (on the right) .
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19. TYROSINE RECOMBINASES
 Tyrosine recombinases break & rejoin
one pair of DNA strand at a time .
 In contrast to the serine recombinases
, the tyrosine recombinases cleave &
rejoin two strands first & only then
cleave & rejoin the other two strands .
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21. CRE RECOMBINASE
 Cre is an enzyme encoded by phage P1 ,
which function to circularize the linear
phage genome during infe-ction .
 The recombination sites on the DNA where
Cre acts , are called lox sites .
 Cre-lox is a simple example of
recombination by the tyrosine recombinase
family , only Cre protein & the lox sites are
needed to complete recombination .
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23. LAMBDA RECOMBINASE
 When bacteriophage lambda infects a host bacterium ,
a series of regulatory events result either in
establishment of the quiescent lysogenic state or in
phage multiplication , a process called lytic growth .
 Establishment of a lysogen requires the integration of
the phage DNA into the host chromosome .
 Likewise , when the phage leaves the lysogenic state
to replicate & make new phage particles , it must
excise its DNA from the host chromosome .
 The analysis of this integration /exision reaction
provided the first molecular insights into site-specific
recombination . 23

24.  To integrate the lambda integrase protein
catalyzes recombination between two
specific sites , known as the att or
attachment sites .
 The att P site is on the phage DNA (P for
phage) & the att B site is in the bacterial
recombinase & the mechanism of strand
exchange follows the pathway described for
the Cre protein .
 Unlike Cre recombination , lambda
integration requires accessory proteins to
help the required Protein-DNA complex to
assemble .
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26. XER RECOMBINASE
 Xer is a member of the tyrosine recombinase family .
 The Xer recombinase catalyzes the monomerization of
bacterial chromosomes & of many bacterial plasmids .
 Xer is a heterotetramer containing two subunits of a
protein called Xer C & two subunits of aprotein called
Xer D .
 Both Xer C & Xer D are tyrosine recombinases but
they recognizes different DNA sequences .
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28. CONCLUSION
 Recombinases are widely used in organisms to
manipulate the structure of genomes & to control gene
expression .
 Different recombinases involved in recombination
processes are RecA , Cre ,Hin , Xer , Lambda
recombinases .
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29. REFERENCES
 Genetics Analysis & Priciples – Robert J.Brooker (4th
Edition)
 Molecular Biology of Gene – Watson (5th Edition)
 www.sciencedirect.com/.../Recombinase
 www.proteopedia.org/.../Recombinase-A
 https://www.ncbi.nlm.nih.gov/.../173646...
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