Transcriptomics Workflow

Instructions to take raw transcriptomics reads and progress to a counts file that can be used in R packages for statistical analysis of differential expression.

To follow this tutorial you will need to have some basic familiarity with working in a unix-like environment. An excellent place to start would be to attend a software carpentry or data carpentry workshop. The VLSCI also have several tutorials for working with unix and/or a HPC environment

Step One - Connect to analysis server

This will usually mean opening a terminal and using ssh to connect. The general format of this command is;

	ssh [email protected]

Step Two - Download your files

Often the sequencing center will provide a url to download raw sequencing data do that a tool like wget can be used to do the download. For example;

	wget -O SequencingData.tar https://data.sequencingcenter.edu.au/download?key

Once you have downloaded the data it is a good idea to keep the original unmodified files somewhere.

Step Three - Unpack the files

How to do this depends on how the files were packaged up. The general tool to use is tar, possibly with options to unzip. Assuming the downloaded data is in a file called Project_ANDI1971.tar we would use this command to unpack.

	tar -xvf Project_ANDI1971.tar

For gzipped files (.gz extension) you can use the -z option to unzip and untar at the same time.

	tar -zxvf Project_ANDI1971.tar.gz

Now move all these raw reads into a folder called raw_data

	mkdir raw_data
	mv Project_ANDI1971 raw_data/

Step Four - Possibly Clean Reads

Check the quality of your sequence data with fastqc. If your reads contain adapter sequences you may need to trim those before going forward.

Step Five - Obtain a reference transcriptome

This might involve a whole extra step of assembling the transcriptome from your own data. Alternatively, a reference might already be available. We assume a reference transcriptome is already available and is called Trinity.fasta. Put this reference transcriptome inside the raw_data folder.

Step Six - Organise Files

This is done with bowtie2. On many HPC systems this can be loaded by doing

	module load bowtie2

Check that it is installed by doing

	which bowtie2

This should return a path showing the location of the bowtie2 program

Now make a directory where we are going to do all the alignment and read counting. Change into this new directory

	mkdir corset
	cd corset

Run the following script to make symbolic links (aliases) for all your raw data files inside the corset directory

file_paths=`find ../raw_data/ -name *.fastq.gz`

for f in $file_paths; do 
	link_name=`basename $f`	
	ln -s $f $link_name
done

Also make a symbolic link for the reference transcriptome

	ln -s ../raw_data/Trinity.fasta transcriptome.fasta

Step Seven - Index the Transcriptome

From within the corset directory run the bowtie2-build command to index the transcriptome

	bowtie2-build transcriptome.fasta transcriptome

Step Seven - Align Reads

Now make a file called bowtie.sh containing the following script. Note that bowtie2 options used in this script were derived from this post on the corset users forum.

This script requires both bowtie2 and samtools to be installed and available. You might need to do module load samtools. The reason we need samtools is to avoid creating a large intermediate file in sam format. Instead we pipe outputs from bowtie2 directly to samtools which converts to the more compacts bam.

for f in `ls *R1*.fastq.gz`; do 
	r1=$f
	r2=${f/R1/R2}
	r12name=${f/R1/both}
	outname=${r12name%.fastq.gz}

	echo "Running bowtie2 on $outname and writing outputs to $outputs.bam"

	bowtie2 --no-mixed --no-discordant --end-to-end --all \
	--min-score L,-0.1,-0.1 \
	--threads 32 \
	-x transcriptome -1 $r1 -2 $r2 | samtools view --threads 4 -b - > $outname.bam

done