Introduction to Bioinformatics - Biol 591

 

SCENARIO
Detecting a stealth anthrax toxin gene
The truth behind Blast

The DNA sequence must tell the story
The standard way of comparing the sequence against others is to run the sequence through the non-redundant (NR) sequences in GenBank, using the Blast facility at NCBI. You do this, and much to your surprise you get the result displayed by clicking here. All you get are hits with E-values well above 1, clearly noise.
sample DNA sequence
vs
toxin DNA sequence

... No similarity!

This result doesn't make sense! This region of the lef gene is highly conserved. If PCR picked up the fragment, then Blast should pick up the sequence.

Maybe you've encountered a novel toxin sufficiently related to lef to permit PCR amplification, but not related enough to match known lef sequences in GenBank. This explanation doesn't sound very compelling, but you can think of no better at the moment, so you translate the DG47 sequence, submit the translated sequence to Blast to search for similar proteins, getting the results displayed by clicking here.

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