the phylogenetics project data revealed! october 4, 2010 oeb 192
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The phylogenetics project data revealed!
October 4, 2010OEB 192
Geographic Distribution
Pathogen = Salmonella Typhi
Why so little turnover?
Letters indicate antibiotic resistant haplotypes
Selection less effective due to “carriers” with no symptoms!
Codon Usage Bias & Selection
October 4, 2010OEB 192
Degenerate Code
Codon Usage BiasNon-random usage of codons
Rickettsia (bacteria) - Winkler 1988
Percent codon usage in family
Codons for each amino acid
Preferredargininecodon
Unpreferredargininecodon
Why non-random codon usage?
Codons correspond to anticodons on tRNAs
Unequal numbers of tRNAs
There are six leucine codons
So, there are six corresponding tRNAs
tRNA copy number differences
E. Coli - Ikemura 1981
Species differ in codon usage biasWithout considering selection, why might different species have
different codon usage patterns?
Massey 2003
6 LeucineCodons
Different GC content alters codon usage
Mutational Bias
232 Eubacterial genomes - Guchte 2006
Is there selection on codon bias?
Four levels of evidence:1. Genome level2. Species level3. Tissue level4. Gene level
Evidence:Non-random patterns of codon usage that are difficult to explain without selection
Selection on codons1. Throughout genomes
Same organism, so not explained by differences in tRNA abundance or mutational bias
2. Between speciesWhy does the pattern decrease from E. coli to Human?
Drummond and Wilke 2008
expressionoptim
al c
odon
s
High Expression level genes have high codon bias
Selection on codons3. Within tissues
Plotkin et al 2004
Red = selectively expressed in human testes
Blue – selectively expressed In human ovaries
Selection on codons4. Within genes
Conserved sites more likely to use preferred codons
Likely tofind Preferred
codon
Likely tofind Unpreferred
codon
Species in alignemnt: Os = Oryza sativa, Pp = Physcomitrella patens, Dd = Dictyostelium discoideum, Hs = Homo sapiens, Dr = Danio reiro, Dm = Drosophila melanogaster, Sc = Saccharomyces cerevisiae, Sp = Schizosaccharomyces pombe, Pf = Plasmodium falciparum Gene = ubiquitin/SUMO-1 like protein
Wang et. al. 2001
Why is there selection on codons?
Movie of Translation: http://www.youtube.com/watch?v=Jml8CFBWcDs&p=22FC96CD697EBE40
Process of Translation
Translational SpeedFaster, so allows more protein to be made
Translational AccuracyIncorrect translation is a waste of resources
Discuss evidence:
Protein FoldingTranslational Speed
Speed tuned to protein folding pace
Translational AccuracyMistranslation induced misfolding
Translational RobustnessProtein folds even with errors
Discuss evidence:
Selection on synonymous sitesOther general reasons for selection on synonymous sites (not specifically codon bias):
Aberrant splicingmRNA instability secondary structure
Weak selectionIs population size dependent – WHY?
Smaller populations are more stochastic:7 heads and 3 tails in 10 coin flips - possiblevs. 70,000 heads and 30,000 tails in 100,000 coin flips – rare!
Weak selectionIs population size dependent
Other possible types of weak selection:
Do humans and mammals have selection on codons?
Much stronger in microbes!Allow us to study these patterns-ascertain mechanistic basis for selection-determine average effect of mutation
Genome sizeNumber of introns
Urrutia & Hurst 2003
1. Could effect dN/dS (or other measures of selection)
...but, It’s very weak selection that doesn’t effect all genes
2. Useful to detect HGT
3. Helpful to consider when building genetically modified organisms
4. Medical applications
– Coleman et al. 2008
Implications of selection on synonymous sites
Pride et al 2006
How is codon bias measured?
Measure tRNA abundance
CAI (Codon adaptation index): measures codon composition relative to reference set (usually highly expressed genes from same genome)
Fop (frequency of optimal codons): Measures the number of preferred codons over the total number of amino acids with synonymous codons
Akashi’s test: Tests how strong the correlation is between conserved sites and preferred codons
Still more: ENC (Effective number of codons)MCB (Maximum Likelihood codon bias