Parse DNA sequences of peak regions from genome

Enrichment analysis of known DNA binding motifs or de novo discovery of novel motifs requires the DNA sequences of the identified peak regions. To parse the corresponding sequences from the reference genome, the getSeq function from the Biostrings package can be used. The following example parses the sequences for each peak set and saves the results to separate FASTA files, one for each peak set. In addition, the sequences in the FASTA files are ranked (sorted) by increasing p-values as expected by some motif discovery tools, such as BCRANK.

library(Biostrings); library(seqLogo); library(BCRANK)
args <- systemArgs(sysma="param/annotate_peaks.param", mytargets="targets_macs.txt")
rangefiles <- infile1(args)
for(i in seq(along=rangefiles)) {
    df <- read.delim(rangefiles[i], comment="#")
    peaks <- as(df, "GRanges")
    names(peaks) <- paste0(as.character(seqnames(peaks)), "_", start(peaks), "-", end(peaks))
    peaks <- peaks[order(values(peaks)$X.log10.pvalue, decreasing=TRUE)]
    pseq <- getSeq(FaFile("./data/tair10.fasta"), peaks)
    names(pseq) <- names(peaks)
    writeXStringSet(pseq, paste0(rangefiles[i], ".fasta")) 

Motif discovery with BCRANK

The Bioconductor package BCRANK is one of the many tools available for de novo discovery of DNA binding motifs in peak regions of ChIP-Seq experiments. The given example applies this method on the first peak sample set and plots the sequence logo of the highest ranking motif.

BCRANKout <- bcrank(paste0(rangefiles[1], ".fasta"), restarts=25, use.P1=TRUE, use.P2=TRUE)
topMotif <- toptable(BCRANKout, 1)
weightMatrix <- pwm(topMotif, normalize = FALSE)
weightMatrixNormalized <- pwm(topMotif, normalize = TRUE)

Figure 2: One of the motifs identified by `BCRANK`

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