How to retrieve fasta sequence after local blast?
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2.2 years ago

Hello, I have created a Blast database using a reference genome. Then, I have performed a local blast search in command line using a gene of interest. I have obtained some hits with the usual Blasting information. Now, I want to extract the exact matching sequence from the blast search with their corresponding start and stop position. I have a .gff file for the genes as well for the reference genome. Can you give me any suggestions on how to do that? Thanks a lot.

genome sequence blast • 4.1k views
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thank you for the useful link

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2.2 years ago
Yannick Wurm ★ 2.5k

If you want to stay in the command line you can:

  • run blast with the text/table output format. This gives you the identifier, start and stop locations for each hit (among other things)
  • using the hit identifier column (the 2nd I believe), extract the FASTA from the blast database using blastdbcmd as highlighted by @genomax. Alternatively, extract the fasta sequence directly from the original reference genome FASTA (genome tools can help you extract just a specific subsequence).

Or if you prefer a point-and-click interface, use SequenceServer to run your BLAST search (FYI - my lab makes the tool; we're proud of it; it's open source). Then in the Download FASTA, XML, TSV section on the left side of the screen (see screenshot below)

  • click on "FASTA of all hits"
  • OR if you just want the aligning segments, click "Alignment of all hits" and retain only the hit sequences from those pairwise alignments.
  • OR select just some of the hits using the [ ] checkboxes & download their entire sequences or aligning parts

The screenshot below shows the download buttons on the left - and shows how next to each hit is a selection checkbox

Yannick

BLAST search results to download FASTA sequence

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Thank you so much for an elaborate reply. Actually, I want to stay on command line and build a pipeline where I can easily blast a gene sequence against a reference and then extract the sequence information from other samples. Right now, I have created a local blast database using the reference genome of interest and also could blast a gene of interest. I was thinking if there is any chance to extract the blast sequence and get their coordinates from the .gff file within command line. What you showed is quite nice but I was thinking of automating these steps within command line. Can you give some feedback on that please?

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I think I still don't understand exactly what you're trying to do. bedtools will likely be your friend - if you get the right columns of the BLAST table output, its effectively a bed file. You can use bedtools to find overlaps between this new .bed file and your existing .gff

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Yes, exactly. That is what I just performed. What I want to do is:

  1. Create a local blast database with the reference genome of interest
  2. Blast the gene of interest against the reference genome
  3. Use the best hit and extract the Start and Stop Position
  4. So you have an interval file
  5. Use that interval file to extract variants from the VCF files of different samples.

Sorry, I should have made it clear before.

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