Outline 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
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
--Genes needed for survival
-Transposon-rich regions
--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