Scripts, Reciprocal Best-Hits BLAST, and some more Python

So we’ve been playing around with scripts of various kinds in the course – the configure.sh script is an example of what’s called a ‘shell script’, and of course the Python programs we’ve been using are also scripts – ‘Python scripts’.

Scripts are just programs written in a scripting language (bash, Python, Perl, etc.) that you run from the command line to perform some function. Shell scripts tend to be “command and control” scripts, scripts that tell other programs what to do, and how; Python scripts tend to be data processing programs, programs that spindle, fold, and mutilate data.

The most important thing to realize is that scripts are just text files, so you can always look at them, and play with them, and modify them to do what you want.

A slight digression

One slightly advanced note: we’ve been ignoring this so far, but you may have noticed that most of the Python scripts have

#! /usr/bin/env python

at the top of the file. This is a “magic” phrase that says, “this file is a Python script. If someone asks you to run it, you can feed it into Python.” The only additional trick here is that you need to tell UNIX that this file is, in fact, a program – to do that, you have to set executable permissions on it, which you can do with this:

chmod +x $filename

e.g.

chmod +x ngs-course/blast/parse-blast-to-csv.py

You can now run this script like so,

ngs-course/blast/parse-blast-to-csv.py results.txt

instead of specifying Python explicitly:

python ngs-course/blast/parse-blast-to-csv.py results.txt

Playing more with BLAST

So, BLAST builds local pairwise alignments between sequences, which (among other things) means that you can have multiple matches between two sequences, and each sequence can match many other sequences. If you want to calculate putative orthologies between species, you need to deal with a confusing situation where there’s more than one match (due to gene duplication and domain duplication) for any sequence.

What you really want is a situation where a (say) mouse gene and a zebrafish gene have a one-to-one relationship, OR the mouse gene has no corresponding zebrafish gene.

(Obviously this has already been done for mouse and zebrafish, but the technique generalizes).

The typical way to deal with this in an ad-hoc manner (which we’ll discuss more on Monday) is to use what’s called “reciprocal best-hits BLAST” approach, where you say:

“I want the mouse gene to be paired with the zebrafish gene such that when I BLAST the mouse gene against the zebrafish proteome, the zebrafish gene is the best hit; and when I BLAST the zebrafish gene against the mouse proteome, the mouse gene is the best hit.”

This is a very restrictive approach that guarantees that you will have either a 1-1 relationship between each mouse & zebrafish gene, OR you will not have any homolog at all.

(For a better approach to all of this, check out OrthoMCL. It’s not really appropriate to use for crappily sequenced genomes, however, and in any case it’s more complicated than I want to get into here :)

Conveniently, I have written a script that does this for you! On your AWS machine, update ngs-course/ by doing

%% cd ~/ngs-course
%% hg pull -u
%% cd -

Now run:

%% python ngs-course/blast/find-reciprocal.py ngs-course/data/mouse.x.zebrafish.csv ngs-course/data/zebrafish.x.mouse.csv > best-hits.csv

(Here’s a cute trick to help explode your head, BTW: try running, instead,

%% python ngs-course/blast/find-reciprocal.py ngs-course/data/{mouse.x.zebrafish.csv,zebrafish.x.mouse.csv} > best-hits.csv

‘best-hits.csv’ will now contain the “reciprocal best hits”. Hooray!

Question: Can you verify that the script works? How?

If you go look at the script, http://bitbucket.org/ctb/ngs-course/src/tip/blast/find-reciprocal.py, it’s a bit more complicated than the previous scripts we’ve been running. This is partly because of the use of functions, which are reusable blocks of code; the logic is also more complicated, and we’re using a new Python data type, dictionaries.

I encourage you to look at the script, but at least for now, I’m more interested in you telling me whether or not it works...

Advanced: Modifying the ‘find-reciprocal’ script to be More Correct

One trick that people often forget about with reciprocal best-hits BLAST is that there can be several equivalently good (i.e. identical bit scores, or e-values) BLAST matches for any sequence. The ‘find-reciprocal’ script ignores this detail – how could you imagine changing the script to deal with that?

Warning: it’s complicated :)


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