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butter: Bowtie UTilizing iTerative placEment of Repetitive small rnas
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SYNOPSIS butter: Bowtie UTilizing iTerative placEment of Repetitive small rnas A wrapper for bowtie to produce small RNA-seq alignments where multimapped small RNAs tend to be placed near regions of confidently high density. LICENSE Copyright (C) 2014 Michael J. Axtell This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. CITATION A preprint describing butter has been deposited at http://biorxiv.org/ Butter: High-precision genomic alignment of small RNA-seq data. Michael J Axtell bioRxiv doi: http://dx.doi.org/10.1101/007427 The manuscript is also, as of this writing (Sep 9, 2014), being prepared for submission to a peer-reviewed journal. AUTHOR Michael J. Axtell, Penn State University, firstname.lastname@example.org DEPENDENCIES perl (version 5; <www.perl.org>) .. installed at /usr/bin/perl samtools <http://samtools.sourceforge.net/> .. installed to your PATH bowtie <http://bowtie-bio.sourceforge.net/index.shtml> .. installed to your PATH OPTIONAL DEPENDENCIES bam2wig <https://github.com/MikeAxtell/bam2wig> .. installed to your PATH. Not required to run butter but is required to make wiggle files from the BAM alignment. bam2wig is also included with the butter download. wigToBigWig <http://genome.ucsc.edu/goldenPath/help/bigWig.html> .. installed to your PATH. Not required to run butter but is required to make bigwig files from wiggle files. INSTALL Install dependencies (see above), and then place the butter script in your PATH USAGE butter [options] [reads.fa/.fasta/.fq/.fastq] [genome.fa] OPTIONS --version: print version and quit --help: print usage and option information, then quit --quiet: suppress progress reports --no_condense: do not condense trimmed reads into non-redundant sequences for bowtie mapping. Preserves original read names, but is (much) slower. --mismatches [integer]: Number of mismatches allowed for a valid alignment. Default 0. Must be either 0 or 1. --aln_cores [integer]: Number of processor cores to use during bowtie alignment phase. Default 1. Must be integer 1-00. --max_rep [integer]: Maximum number of possible placements for a multi-mapped read being guided by density. Such reads with a number of placements more than --max_rep will be reported as unmapped and have custom tag XY:Z set to M. Default: 1000. Must be integer of at least 6. --ranmax [integer]: Maximum number of possible placements for a multi-mapped read that can't be guided by density. Such reads with a number of placements more than --ranmax will be reported as unmapped and have custom tag XY:Z set to O. Default: 3. Must be integer of 1 or more. --adapter [string]: 3' Adapter sequence to trim. Must be 8 or more ATGCatgc characters. If specified, 3' adapter trimming is enabled. --bam2wig [string]: Options for generating wiggle and bigwig coverage file(s) from the final BAM alignment. Can be 'combined', 'degradome', 'strandspec2', or 'none'. Defaults to 'combined'. Combined merges coverage on both strands to a single positive value. Degradome creates two separate tracks for the plus and minus strands, each tallying just the coverage at 5' ends. strandspec2 creates two files, one for each strand, tallying total coverage. None creates no wiggle/bigwig files. INPUT FILES Small RNA-seq data Files must be in FASTA, FASTQ, or colorspace-fasta format. File extensions must be used to indicate the format. .fasta and .fa are acceptable for FASTA files. .fastq and .fq are accetpable for colorspace files. .csfasta must be used for color-space files. There is no support for paired-end reads. Colorspace data are assumed to conform to colorspace-FASTA specifications (beginning with a nucleotide, followed by a string of colors [0,1,2,3] or ambiguity codes [.]. If trimmed colorspace data are provided, it is assumed that the 'hybrid' color at the 3' end has been removed. If colorspace data are trimmed by butter, the hybrid color at the 3' end will be removed (see below). Reference genome File must be in FASTA format. Chromosome names will be truncated after the first white-space encountered. butter will search for the expected bowtie indices in the same directory as the genome file. If the input format is FASTQ or FASTA, butter will expect the bowtie indices to have the form [your_genome].[ebwt], where 'your_genome' is the name of your genome file, and [ebwt] represents the six distinct file extensions for bowtie genome index files. If your input data is colorspace, butter will expect the genome indices to have the form [your_genome].cs.[ebwt] instead. The .cs serves as a reminder that the indices are built in colorspace. If the expected bowtie indices are not found, butter will attempt to use bowtie-build under default parameters to build them. METHODS Adapter trimming - FASTA For each read, the 3'-most exact match to the supplied adapter sequence (via option --adapter) is found and trimmed off. Trimmed data shorter than 15nts are suppressed from output. In addition, reads with non ATGCatgc characters after trimming are also suppressed. Comment lines in the original file are ignored, and will not be output to the trimmed file. Adapter trimming - FASTQ Identical to trimming for FASTA data, with the addition of trimming the quality values to the same length as the trimmed sequence data. Adapter trimming - Colorspace-FASTA The input --adapter sequence is converted to colorspace. Then, for each read, the 3'-most exact match to the color-string is found. Trimming takes off the matched color string, as well as the ambiguous color left at the end .. this is the hybrid color formed by the di-base created by the last nucleotide of the small RNA and the first nt of the adapter. When mapped using bowtie with the --col-keepends option, this trimming results in the alignment of the full length small RNA. Read Condensation For the purpose of bowtie mapping, the trimmed reads are condensed such that each unique small RNA sequence is represented only once. In the process, the reads are re-named. Condensaiton can save considerable CPU time, and the re-naming of the reads saves substantial memory (because each read name does not have to be stored by the script for later output). The condensed reads are written in FASTA or .csfasta format (e.g. the quality values from FASTQ files are ignored). This is justified because the bowtie alignment parameters being used ignore quality values. The condensed reads are written to a file in the working directory with the name [your_reads]_condensed.(cs)fasta. The condensed read names follow a simple system. Consider an example read name: >my_reads_758883_12 The '758883' indicates that this is unique sequence number 758883 (arbitrarily ordered). The '12' indicates that there were 12 reads in the input file with this sequence. The option --no_condense turns off read condensation, so the original read names are preserved. This option is much slower. If option --no_condense is used for an input file in FASTQ format, a FASTA version of the reads will be written to disk. Alignments and placements Four iterations of bowtie are called successively on the reads after adapter trimming (if applicable) and condensation (if applicable) and conversion from FASTQ to FASTA format (if applicable, see above). The first iteration uses bowtie setting -m 1 to limit alignments to reads with only one unique possible position in the reference genome. This output stream is parsed to retain unmapped reads, and reads with unique alignments. Using bowtie's --max option, reads with more than 1 possible alignment are written to a temporary FASTA or csFASTA file. After this, the densities of the uniquely placed reads are tallied, genome-wide, using a sliding window of 250 nts, and a step size of 50 nts. The density in each window is simply the sum of all of the read-depths at each nt in the window (e.g., 'area under curve'). The second bowtie iteration uses the multi-mapped reads set aside in the first iteration for a run with bowtie -m set to 2. Thus, only reads with 2 possible placements are output. After calculating the densities, both possible placements for each read that had 2 possible locations are analyzed. Each location falls into 5 different windows .. the window with the maximum score is taken as the existing density of each placement. In cases where all possible placements have an exisiting density of zero, the choice of which placement to retain is simply random. If one possible placement has existing density and the other doesn't, the retained placement will be that which is next to existing density. If both possible placements have existing density, the retained placement will be selected based on probabilities dictated by the relative density abundances among the two choices. For example, if placement one had a maximum density score of 70, and and placement two had a maximum density score of 30, the probability of placement one being retained is 70 / (70 + 30) [e.g. 70%] and the probability of placement two being retained is 30 / (70 + 30) [e.g. 30%]. As in the first iteration, reads with multi-mappings higher than the -m set point are written to a temporary FASTA or csFASTA file for analysis in the next iteration. After the process is repeated for two more iterations .. the densities are updated with the new data, and multi-mapped reads that remain are re-mapped iteratively. Iteration three captures reads with 3-5 possible placements, and iteration four captures reads with between 5 and --max_rep number of placements (default is 1000). De-condensation Unless run with option --no_condense, reads must be de-condensed. This occurs while writing sorted BAM files, the non-redundant queries are de-condensed, such that there is an alignment line for each copy of that sequence present in the input data. For instance, with our example read from above (my_reads_758883_12), there were 12 copies. So, somewhere in the output BAM file there will be twelve reads .. my_reads_758883_12, my_reads_758883_11, my_reads_758883_10 ... all the way down to my_reads_758883_1. Note that, for reads with more than 1 potential placement, each de-condensed read is considered separately. Because placement of multi-mapped reads can be either random or probabilistic (see above 'Placement'), this means that not all copies of an identical sequence will necessarily be placed at the same location. Merging After all iterations of placements and density calculations have completed, the resulting sorted bam files are merged to a single final bam alignment, and the intermediate bam files deleted. Temporary files butter writes a number of temporary files to disk during a run .. intermediate BAM files, and temporary FASTA (or csFASTA) files. These will all be deleted at the completetion of the run. OUTPUT The output is a single BAM alignment file sorted by chromosomal position. The BAM header includes lines describing the run. butter adds three custom tags for each alignment. All reads are in the alignment file, including reads that were unmapped (unmapped reads have bit 0x4 set in the SAM FLAG field, per SAM specification). Other custom tags not described below are from bowtie .. see bowtie documentation for their meaning. Custom tag XX:i: indicates number of valid placements for the read (of which only one is being shown) Custom tag XY:Z: indicates how the reported placement was selected. U: uniquely mapped, P: multi-mapped and placed due to clustering, R: multi-mapped and randomly placed, N: unmapped, M: Multi-mappings exceeded setting --max_rep, so no placement performed, O: multi-mapped with no density-based placement possible, number of locations exceeded setting --ranmax, so no placement performed. Custom tag XZ:f: indicates the probability that the given read came from the reported position, based on the butter iterative density analysis. Set to 1 for reads with XY:Z: of M, U, N, and O. Wiggle and bigwig files Depending on setting of option --bam2wig and the availability of the bam2wig and wigToBigWig programs, butter will also create wiggle and bigwig coverage files for easier use on a genome browser.