Table of contents

  • Expected learning outcomes
  • BLAST+ overview
  • Exercise 1: BLAST+ installation and using a pre-formatted a database
  • Exercise 2: performing a basic BLASTp search
  • Exercise 3: modifying the defaults and output types
  • Exercise 4: creating a custom database
  • Exercise 5: BLASTing against a remote database
  • Exercise 6: Extracting hits from a BLAST database
  • Converting output format types

Expected learning outcomes

The objective of this lab is to get accustomed with performing BLAST searches from the command line. We will cover basic BLAST searching, modifying parameters, modifying output files, creating your own database, online searching and hit sequence extraction.

BLAST+ overview

The command line (UNIX or Windows) version of BLAST is named BLAST+. You can download it from here. The user manual for BLAST+ is here and contains instructions on installation of the command line tools. This covers all of the functionality of BLAST+. Here, only a subset of features and basic usage will be covered.

Exercise 1: BLAST+ installation and using a pre-formatted a database

In order to run BLAST+ we must download it, install it and set our Path to point to it. First go here and download the appropriate version. For our Ubuntu installation we want the ncbi-blast-2.2.27+-x64-linux.tar.gz file. Once downloaded, extract to the software folder (found within the home/wme_jan2013 folder). You should now have a folder called ncbi-blast-2.2.27+

To run the programs from the command line we need to place them in our Path. The path tells  the computer the location of each program so that we do not have to type out the location each time. Open a new terminal and type the following commands:

echo PATH=$PATH:~/wme_jan2013/software/ncbi-blast-2.2.27+/bin >>.bashrc

echo export PATH >>.bashrc

source .bashrc

This tells the operating system to add the directory containing the BLAST programs to our path.

Test that this works by typing

blastn

If you get an output like “BLAST query/options error: Either a BLAST database or subject sequence(s) must be specified” then BLAST+ is installed correctly

Most often when using BLAST+ we wish to ether create a custom database or a local version of a pre-made database to BLAST against. Several pre-made databases are provided by NCBI here. These are often very large and are split into several files (take a look at the amount of files needed for nr). As these would take a long time to download, I have premade a database for you equivalent to the pdb database. This is a dataset containing all the protein sequences associated with PDB files (protein structure files).

Download the zip file here. Inside there is a folder called db which contains many files, all of which begin with ‘pdbaa’. Each of these allows for a different type of search to be performed. You will notice each has a ‘p’ after the period. This denotes that a protein database has been created. We will see later how to create this database ourselves.

Exercise 2: performing a basic BLASTp search

BLAST+ search strategies are run by typing the type you want on the command line followed by the input options. This includes blastn, blastp, blastx, tblastn and tblastx. Ensure you know which search strategy is appropriate for your data and database type.

We will now search a few protein sequences against the database and retrieve the results. Within the zip file you downloaded you will see a file named proteins.fasta. This is a fasta file containing two sequences. We will perform a default BLASTp search on these to find out what proteins they are. The advantage of BLAST+ is that we can run BLASTp once using this file as an input and it will perform a search on all sequences within, meaning we do not have to do each sequence individually.

Navigate to the folder containing proteins.fasta. The db folder should be in this folder too. Type the following on the command line:

blastp -query proteins.fasta -db db/pdbaa -out proteins_blastp.txt

This will perform a blastp search, using all the sequences in proteins.fasta as queries, using the pdbaa database and output the results to proteins_blastp.txt. You will see the output looks quite like the website output with the overview first and the individual alignments next.

Q. What sequences do we appear to have?

 

There are many ways to modify how this is run. Type

blastp -help

This will print to the screen a large amount of text, detailing all the flags (options) we can change during the BLAST search. We will modify some of these now.

 

Exercise 3: modifying the defaults and output types

Often if we are working with many sequences we want to make it easier to get the best results in an easy to read format. We can do this by limiting the number of results returned. Often this is performed by changing the number of alignments displayed and/or the e-value cut-off.

Lets run BLASTp, keeping all the overviews but only displaying the top hit alignment. We change the number of alignments displayed with the -num_alignments flag. To keep only the top hit alignment we can use

blastp -query proteins.fasta -db db/pdbaa -out proteins_blastp_1align.txt -num_alignments 1

If you look in this file we can see that all the descriptions are retained but now we only have 1 alignment per query sequence. Another way to limit the results is to set an e-value cut-off. In the help output of each program you can see the default e-value (listed under ‘general search options’). You can see it is quite high (10). We will retain only those hits with an e-value of 1e-30 or higher. We use the -evalue flag for this. Lets combines the above alignment display restriction with an evalue restriction. This done by typing:

 

blastp -query proteins.fasta -db db/pdbaa -out proteins_blastp_1align_1e-30.txt -num_alignments 1 -evalue 1e-30

You can see now that we have a smaller number of hits, hopefully including only those we are certain are likely to be correct.

Often we dont need the output alignments but want all the details of each hit (e-value, bit score, percent identity etc) on 1 line. This is achieved by changing the output type. So far we have used the default output type but we can change this with the -outfmt flag. There are 11 output types (listed in the help output) but we shall use type 6: tabular output. This gives you a line per hit with 13 columns:

Query id
Subject id
% identity
alignment length
mismatches
gap
openings
query start
query end
subject start
subject end
e-value
bit score

This saves a lot of space in the output but does not separate queries into separate sections. We no longer need to limit the number of alignments with this output format (as none are displayed) but we may still want to limit the e-value. We can do this by typing:

blastp -query proteins.fasta -db db/pdbaa -out proteins_blastp_1e-30_table.txt -evalue 1e-30 -outfmt 6

We can see the output is much more compressed with queries all together, only separated by the name in the first column. You may notice however that the subject name (column 2) is truncated, with the information we want having been cut off. This is a known problem in the tabular output that supposedly NCBI are working on. You can see we do happen to have the GI included in the description which we can search the NCBI website for. When we create our own database we may wish to try have short names so that this is less of a problem. There is a way to switch between some output types even after you have run the analysis which I will outline briefly later.

 

A small note ( we will not do an exercise on this): There are different types of BLAST searches within the basic types. For example in blastn you can perform standard blastn, megablast or discontiguous megablast. These options are changed with the -task flag. Note that megablast is the default for blastn, not standard blastn.

Exercise 4: creating a custom database

Often we do not want to use all of NR or any of the pre-made databases supplied by NCBI. We may want to use a custom database of only species or genes we are interested in. If we have a fasta format file (unaligned) of these sequences we can create a database from this with the makeblastdb command. Lets create the pdb amino acid database from a fasta file, resulting in the database we already used.

Create a new folder called db2. Copy the file pdbaa.fasta from the db folder to the db2 folder. Navigate into the db 2 folder and create a protein database by typing:

makeblastdb -in pdbaa.fasta -title pdbaa -dbtype prot -out pdbaa -parse_seqids

The -in flag states the fasta file to create the database from, the -title flag gives the database a title, -dbtype says whether it is protein (prot) or nucleotide (nucl), -out is the name of the database and -parse_seqids states we want to retain the full names of each sequence. You will not see in db2 we have exactly the same files as in the db folder. We can use this as our datase in exactly the same way we did for the original pdbaa database.

 

Exercise 5: BLASTing against a remote database

Instead of having to download the entirety of NR or other NCBI databases, we can BLAST against the version held on the website. This ensures we have the most up to date version but is also significantly slower. We use the -remote command to do this. Lets BLAST out sequences against NR held on the NCBI website by typing:

blastp -query proteins.fasta -remote -db nr -out proteins_nr.txt -outfmt 6 -evalue 1e-30

 

Exercise 6: Extracting hits from the BLAST database

Once we have our BLAST results we may wish to go back and get the sequences for the hits from the database. For this we require a file of the sequence names of the hits (or parts of it, such as the section output from type 6 output) and the database used for the original BLAST (which must have been created with the -parse_seqids flag). Lets take 2 sequences from our hits against the pdbaa database. Copy the following into a file called hits.txt

gi|1942986|pdb|1OCC|A
gi|40889823|pdb|1V54|A

This is portions of 2 names of sequences we found to be good hits to our first query sequence. We will use the blastdbcmd program to get these sequences from the pdbaa database. Type:

blastdbcmd -db db/pdbaa -dbtype prot -entry_batch hits.txt -outfmt %f -out hits.fasta 

The -db, -dbtype and -out we have seen before, -entry_batch is the file containing the sequence names and -outfmt here says we want fasta formatted sequences (%f). If you now open hits.fasta you should see the 2 sequences we requested.

 

Converting output format types

If you wish to change output formats after you have run a BLAST search we can use blast_formatter. This requires that the original run used -outfmt 11 (archive type) and the database was made with the -parse_seqids flag. If we ran a BLAST such as

blastp -query proteins.fasta -db db/pdbaa -out protein_archive.txt -outfmt 11

We could retrieve the results in out format 6 by typing

blast_formatter -archive protein_archive.txt -outfmt 6 -out proteins_tabular.txt

Thus a good way to run BLAST+ is to use -outfmt 11 and then after that use blast_formatter to change the output to different formats as needed.

 

One thought on “BLAST+ laboratory

  1. marianna

    Under exercise 3 don’t you mean ‘We will retain only those hits with an e-value of 1e-30 or lower’?

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