Agricultural Microbiology

1. Bacterial genetics:
genetic recombination-
transformation, conjugation and
transduction; plasmids and
transposon.
Dr. S. PARTHASARATHY, M.Sc. (Agri)., Ph.D.
Assistant Professor (Plant Pathology)

2. Genetic recombination
Transfer of DNA from one organism (donor) to another organism (recipient).
The transferred donor DNA may then be integrated into the recipient's nucleoid by
various mechanisms (Homologous, Non-homologous)
A. Natural mechanisms of genetic homologous recombination in bacteria
include
a. Transformation (Grifith 1928 – Diplococcus pneumonia)
b. Transduction (Lederberg and Zinder 1952 – Salmonella typhi
c. Conjugation (Lederberg and Tatum 1946 – Escherichia coli)
In transformation, a bacterium takes up a piece of DNA floating in its environment.
In transduction, DNA is accidentally moved from one bacterium to another by a virus.
In conjugation, DNA is transferred between bacteria through a tube (pili) between cells.
Transposable elements are chunks of DNA that "jump" from one place to another. They
can move bacterial genes that give bacteria antibiotic resistance or make them disease-
causing.
B. Natural mechanisms of genetic heterologous recombination in bacteria
include Plasmid, Transposon

4. Transduction

6. Transformation

8. Conjugation

11. Plasmid
• Extra-chromosomal genetic elements. Plasmid is a circular
double stranded DNA having several to 100 kbp.
• Plasmids replicate autonomously and they code for functions
which are normally not indispensable.
• In some cases, plasmids may be transferred from one cell to
another and thus may carry sets of specialized genetic
information through a population,
• e.g. Drug resistance plasmids (R factors) may render diverse
bacteria resistant to antimicrobial drugs.

12. • Plasmids can be removed from the host cell in the process
of curing
• Curing may occur spontaneously or may be induced by
treatments such as ultraviolet light. Certain plasmids,
called episomes, may be integrated into the bacterial
chromosome.
• Special plasmid called a fertility (F) factor plays an important
role in conjugation.
• Genes that encode a series of bacteriocins are also found on
plasmids. Bacteriocins are bacterial proteins capable of
destroying other bacteria.

13. Classification of Plasmids
Transfer properties
Conjugative plasmids: Conjugative plasmids are those that
mediated conjugation. These plasmids are usually large and have
all the genes necessary for autonomous replication and for
transfer of DNA to a recipient (e.g. genes for sex pilus).
Non-conjugative plasmids: Non-conjugative plasmids are those
that cannot mediate conjugation. They are usually smaller than
conjugative plasmids and they lack one or more of the genes
needed for transfer of DNA. A non-conjugative plasmid can be
transferred by conjugation if the cell also harbors a conjugative
plasmid.

14. Based on function:
Fertility F-plasmids:
• Which contain tra genes. They are capable of conjugation and
result in the expression of sex pilli.
Resistance (R) plasmids:
• Which contain genes that provide resistance
against antibiotics or poisons.
• Historically known as R-factors, before the nature of plasmids
was understood.
Col plasmids:
Which contain genes that code for bacteriocins, proteins that can kill
other bacteria.
Degradative plasmids:
Which enable the digestion of unusual substances,
e.g. toluene and salicylic acid.
Virulence plasmids:
Which turn the bacterium into a pathogen. Plasmids can belong to
more than one of these functional groups

16. Transposons
• Transposable elements. Transposable elements, also known
as transposons, are segments of DNA that move about within the
chromosome and establish new genetic sequences.
• First discovered by Barbara McClintock in the 1940s,
• Transfer of transposons can occur between one plasmid to
another, or between plasmid and chromosome within a bacterial
cell.
• The process is called transposition. Transposons carry genes for
specialized functions, e.g. antibiotic resistance.
• Transposons do not contain genetic information for their
replication.

17. Bacteria contain two types of transposons
1. Composite mobile genetic elements that are larger than IS
elements and contain one or more protein-coding genes in
addition to those required for transposition
2. Non composite mobile genetic elements are those which lack
IS elements on its ends e.g. is Tn3