Transcript What are Transposons?

What are Transposons?
• “Selfish DNA”
• Interspersed repeats
• “move” in the genome
Bacterial Insertion
Sequences
• Transpositions inactivate essential genes, killing
the host and the IS element it carries
• Some transposed sequences enter nonessential
regions of the genome allowing regions of genes
to survive
Transposase
• Required for transposition of IS element to new site
• Target site direct repeat sequence are immediately adjacent
to both ends of the inserted elements
Transposase Functions
3’ of Is
element to
5’ ends of
cut donor
DNA
McClintock’s Discovery
• Spontaneous mutations in maize affect
production of enzymes required for pigment
2 Movable Elements
1. Activator elements
Encodes mRNA for transposase protein.
-recognizes the terminal repeats and catalyzes
transposition to a new DNA site
2. Dissociation elements
-deletion mutant of Activator that has lost the
necessary sequences to move on its own
Results of Transposons
• Can result in an increase in the number of a
transposon if it occurs during S phase of cell
cycle
• after a DNA transposon leaves a gene, the
resulting gap will probably not be repaired
correctly
• Transposition in germ cells to their new sites is
passed on to succeeding generations
Phytophthora infestans
• Oomycete – aka “water mold” – fungus-like
eukaryotic organism “pseudofungus”
• Cause of potato blight, which brought about the
Irish Potato Famine
Genome organization
• Transposon-poor regions
o Genes needed for survival
• Transposon-rich regions
o Genes used for pathogenesis
• 74% of genome is transposon-rich
• Most transposons in genome are inactive
Plant Immune System
• Pathogen-associated Molecular Patterns (PAMPs)
causes response of PAMP-triggered Immunity (PTI)
• Pathogens produce effectors to suppress PTI
• Plants produce R proteins to suppress effectors
• High selective pressure on effector genes leads to
rapid evolution
RNA Silencing
• P. infestans uses sRNA to restrict the activity of
transposons
Effector Silencing
• Proximity to transposons
leads to silencing of
effector genes
• Caused by formation of
heterochromatin at and
around transposons
• Heterochromatin
spreads ~300 to 600
bases from transposon
• 563 RXLR effectors
• 35 within 300bp of transposons
• 106 within 600bp
• 283 within 2kb