SPRY-SARUS motif scanner
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README.md

SPRY-SARUS

SPRy-SARUS stands for Straightforward yet Powerful Rapid SuperAlphabet Representation Utilized for motif Search.

This is a simple tool which uses superalphabet approach presented by [Pizzi, Rastas & Ukkonen; 2007] to scan a given set of sequences for (di)PWM hits scoring no less than a given threshold.

Key features:

  • provides bed + plain text output
  • supports both classic mono- and dinucleotide position weight matrices
  • can output raw scores or scores converted to P-values

Usage

SPRY-SARUS has fairly simple command line format, accepts weight matrices in plain text files (with log-odds or similarly transformed additive weights) and DNA sequences in multifasta.

SPRY-SARUS is written in Java (and requires Java 1.8 or more recent). You can get it as the jar file. You can experiment with sample PWMs and sequences in examples folder.

The proper command line format is printed if SARUS is executed w/o arguments:

java -cp sarus.jar ru.autosome.SARUS

or in a shorter form

java -jar sarus.jar

You may want to provide Java with more memory in case of large sequence sets, e.g. 2 gigabytes:

java -Xmx2G -cp sarus.jar ru.autosome.SARUS <more parameters here>

Using dinucleotide version

The dinucleotide version can be used in a similar way (with the same command-line parameters):

java -cp sarus.jar ru.autosome.di.SARUS

For example:

java -cp sarus.jar ru.autosome.di.SARUS SP1_peaks.mfa SP1_example.dpwm besthit > result.log

[NOTE!] The dinucleotide version is located in ru.autosome.di package, and this is the only difference in command line format.

Input data

SPRY-SARUS scans given sequences for occurrences of the given motif. For each sequence it acts independently (no internal parallelization is implemented).

SPRY-SARUS operates in two different modes.

In the first mode it looks for the only best occurrence (besthit) of the motif in each sequence:

java -cp sarus.jar ru.autosome.SARUS <sequences.multifasta> <weight.matrix> besthit

Another option is to return all motif occurrences in a sequence for which PWM score exceeds the specified threshold:

java -cp sarus.jar ru.autosome.SARUS <sequences.multifasta> <weight.matrix> <threshold>

The arguments have self-speaking names. The multi-FASTA with sequences and the weight matrix file are specified by the corresponding filenames.

Sequences can be also be passed via the standard input (stdin) by specifying minus (-) instead of the corresponding filename.

Weight matrix format

The weight matrix can be given either with or without the header line (starting from >).

By default each line in the matrix file corresponds to a single position of a motif, i.e. each line should contain 4 (or 16) elements for PWM (diPWM).

The nucleotide order for mono-PWMs is alphabetical A-C-G-T. The dinucleotide order for di-PWMs is also alphabetical AA-AC-AG-..-TT.

Please note, that SARUS can use raw ChIPMunk (but not ChIPHorde) output extracting the resulting motif right from the log-file (if the ChIPMunk output was redirected into a file as suggested in ChIPMunk guide, see the ChIPMunk website).

Options

The first three arguments are mandatory. The next arguments are options that modify how the input is treated and what should be modified in the output:

  • --transpose suggests SARUS to use the transposed file format for the matrices (nucleotides or dinucleotides as rows, motif positions as columns).
  • --direct or --revcomp forces single strand scanning mode to scan only the direct strand or only reverse-complementary strand
  • --skipn can be used to skip words with an unknown (N)-nucleotides.
  • --precision N should be used to round the resulting values (either raw PWM score or P-values) up to N digits after the floating point. Doesn't affect the internal precision of calculations.

Sequence motifs with long flanking sequences can hang over extremely short sequences. By default SARUS ignores motif occurrences which are not located totally inside of a particular sequence. There are two options to override this behavior:

  • --show-non-matching In the besthit mode a user normally expects exactly one result for each input sequence. But for sequences shorter than the motif SARUS can't find the best occurrence and outputs nothing. With this option SARUS outputs a fictional result instead (which can be useful to simplify parsing of the resulting file). The fictional "occurrence" has a score of -∞ and is located at a virtual position -1 with zero occurrence length.
  • --add-flanks Instead it is possible to extend each sequence by poly-N flanking sequences long enough to embrace putative sites. Given the flanking sequences are added, each sequence will have the "besthit" occurrence. Note, that the occurrence coordinates can point to the outside of sequence! Occurrence start can be negative, and occurrence endpoint can be higher than sequence length.

Please note, that all the options (except for the filenames) should be given in lowercase letters.

Scores and P-values

By default SPRY-SARUS reports PWM scores of motif occurrences. Often it's more convenient to use P-values corresponding to the scores. To convert scores to P-values, SARUS relies on a precalculated mapping between score thresholds and P-values. Such mapping can be obtained by MACRO-PERFECTOS-APE using ru.autosome.ape.PrecalculateThresholds with a uniform grid over logarithmic P-values with any given level of precision.

The P-values scanning requires the file with the mapping using --pvalues-file FILE option.

The output occurrence scores can be provided as P-values or negative logarithm of the P-values (logpvalue := -log10(P-value)) with the following option: --output-scoring-mode MODE

where MODE can be one of: score/pvalue/logpvalue (default: score).

For example, to find the best occurrences and report their logarithmic P-values the following command-line can be used:

java -cp sarus.jar ru.autosome.SARUS sequences.mfa motif.pwm besthit --pvalues-file threshold_pvalue_mapping.txt --output-scoring-mode logpvalue

Also, you can tell SPRY-SARUS to treat occurrence threshold as a P-value or a logarithmic P-value: --threshold-mode MODE. For example, to find sites with P-value not greater than 0.0001:

java -cp sarus.jar ru.autosome.SARUS sequences.mfa motif.pwm 0.0001 --pvalues-file threshold_pvalue_mapping.txt --threshold-mode pvalue

Output in bed-format

With the option --output-bed it is possible to output motif occurrences in bed-format. BED-6 format includes: chromosome name, start and end positions in [closed; open) 0-based indexing, interval name, score (or P-value/logarithmic P-value), strand.

Chromosome name and sequence position is inferred from the multi-FASTA header lines. To generate correct genome-wide coordinates each sequence should be named as >chrName:posStart-posEnd

Note, that

bedtools getfasta generates headers in matching format.

The posEnd value can be omitted. SPRY-SARUS ignores header content after posStart. If the posStart is missing, it's set to 0 by default. Chromosome name and position shouldn't contain spaces and other whitespace characters.

Interval name in the 4-th column of bed-file is combined based on the motif name and the sequence name. By default the motif name is inferred from the PWM filename but can be redefined using the --motif-name NAME option.

In bed format it's natural not to output sequence names that belong to different sequences of the multi-FASTA. In default output format (plain text) it's natural to output the names - this is the default behaviour. You can force SPRY-SARUS both to suppress sequence names output in default mode using --suppress and not to suppress them in BED-format using --no-suppress.

Please note, that output is not sorted (because of matches on different strands). Consider using bedtools sort before supplying the output to other bed-based tools.

Output format

The default output format is fairly simple showing the sequence header (via > as in the input multifasta file), the PWM score, the position and the strand orientation of the motif occurence.

An example of the command line (based on example data from the webpage):

java -jar sarus.jar SP1_peaks.mfa SP1_example.pwm besthit

The messages are printed to STDERR and the result is printed to STDOUT so it is possible to redirect the result into a file:

java -jar sarus.jar SP1_peaks.mfa SP1_example.pwm besthit > result.log

When --output-bed option is specified, the resulting output follows BED-6 format specification. The interval name is composed of the motif name and the sequence name separated by a semicolon, e.g.: GATA1;chr1:100234-100567.