Atropos was designed to work with colorspace reads from the ABi SOLiD
sequencer. Colorspace trimming is activated by the
option (or use
-c for short). The input reads can be given either:
- in a FASTA file
- in a FASTQ file
- in a
.qualfile (this is the native SOLiD format).
In all cases, the colors must be represented by the characters 0, 1, 2,
3. Example input files are in the atropos distribution at
.qual file format is
automatically assumed if two input files are given to atropos.
In colorspace mode, the adapter sequences given to the
-g options can be given both as colors or as nucleotides. If
given as nucleotides, they will automatically be converted to
colorspace. For example, to trim an adapter from
solid.qual, use this command-line:
atropos -c -a CGCCTTGGCCGTACAGCAG solid.csfasta solid.qual > output.fastq
In case you know the colorspace adapter sequence, you can also write
330201030313112312 instead of
CGCCTTGGCCGTACAGCAG and the result
is the same.
Ambiguity in colorspace¶
The ambiguity of colorspace encoding leads to some effects to be aware
of when trimming 3’ adapters from colorspace reads. For example, when
trimming the adapter
AACTC, atropos searches for its
0122. But also
GGTCT have an encoding of
0122. This means that effectively four
different adapter sequences are searched and trimmed at the same time.
There is no way around this, unless the decoded sequence were available,
but that is usually only the case after read mapping.
The effect should usually be quite small. The number of false positives is multiplied by four, but with a sufficiently large overlap (3 or 4 is already enough), this is still only around 0.2 bases lost per read on average. If inspecting k-mer frequencies or using small overlaps, you need to be aware of the effect, however.
Double-encoding, BWA and MAQ¶
The read mappers MAQ and BWA (and possibly others) need their colorspace
input reads to be in a so-called “double encoding”. This simply means
that they cannot deal with the characters 0, 1, 2, 3 in the reads, but
require that the letters A, C, G, T be used for colors. For example, the
0011321 would be
AACCTGC in double-encoded
form. This is not the same as conversion to basespace! The read is still
in colorspace, only letters are used instead of digits. If that sounds
confusing, that is because it is.
Note that MAQ is unmaintained and should not be used in new projects.
BWA’s colorspace support was dropped in versions more recent than 0.5.9, but that version works well.
When you want to trim reads that will be mapped with BWA or MAQ, you can
--bwa option, which enables colorspace mode (
-d), primer trimming (
-t), all of which are
required for BWA, in addition to some other useful options.
--maq option is an alias for
To cut an adapter from SOLiD data given in
solid.qual, to produce MAQ- and BWA-compatible output, allow the
default of 10% errors and write the resulting FASTQ file to
atropos --bwa -a CGCCTTGGCCGTACAGCAG solid.csfasta solid.qual > output.fastq
Instead of redirecting standard output with
-o option can
be used. This also shows that you can give the adapter in colorspace and
how to use a different error rate:
atropos --bwa -e 0.15 -a 330201030313112312 -o output.fastq solid.csfasta solid.qual
This does the same as above, but produces BFAST-compatible output, strips the _F3 suffix from read names and adds the prefix “abc:” to them:
atropos -c -e 0.15 -a 330201030313112312 -x abc: --strip-f3 solid.csfasta solid.qual > output.fastq
Quality values of colorspace reads are sometimes negative. Bowtie gets confused and prints this message:
Encountered a space parsing the quality string for read xyz
BWA also has a problem with such data. Atropos therefore converts
negative quality values to zero in colorspace data. Use the option
--no-zero-cap to turn this off.
Sequence Read Archive¶
The Sequence Read Archive provides files in a special “SRA” file format. When
fastq-dump program from the sra-toolkit package is used to convert
.sra files to FASTQ format, colorspace reads will get an extra
quality value in the beginning of each read. You may get an error like this:
atropos: error: In read named 'xyz': length of colorspace quality sequence (36) and length of read (35) do not match (primer is: 'T')
To make atropos ignore the extra quality base, add
your command-line, as in this example:
atropos -c --format=sra-fastq -a CGCCTTGGCCG sra.fastq > trimmed.fastq
When you use
--format=sra-fastq, the spurious quality value will be removed
from all reads in the file.