Quality trimming in R using ShortRead and Biostrings

March 3, 2010
By

[This article was first published on Jermdemo Raised to the Law, and kindly contributed to R-bloggers]. (You can report issue about the content on this page here)
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I wrote an R function to do soft-trimming, right clipping FastQ reads based on quality.

This function has the option of leaving out sequences trimmed to extinction and will do left-side fixed trimming as well.

#softTrim
#trim first position lower than minQuality and all subsequent positions
#omit sequences that after trimming are shorter than minLength
#left trim to firstBase, (1 implies no left trim)
#input: ShortReadQ reads
# integer minQuality
# integer firstBase
# integer minLength
#output: ShortReadQ trimmed reads
library("ShortRead")
softTrim<-function(reads,minQuality,firstBase=1,minLength=5){
qualMat<-as(FastqQuality(quality(quality(reads))),'matrix')
qualList<-split(qualMat,row(qualMat))
ends<-as.integer(lapply(qualList,
function(x){which(x < minQuality)[1]-1}))
#length=end-start+1, so set start to no more than length+1 to avoid negative-length
starts<-as.integer(lapply(ends,function(x){min(x+1,firstBase)}))
#use whatever QualityScore subclass is sent
newQ<-ShortReadQ(sread=subseq(sread(reads),start=starts,end=ends),
quality=new(Class=class(quality(reads)),
quality=subseq(quality(quality(reads)),
start=starts,end=ends)),
id=id(reads))

#apply minLength using srFilter
lengthCutoff <- srFilter(function(x) {
width(x)>=minLength
},name="length cutoff")
newQ[lengthCutoff(newQ)]
}

To use:

library("ShortRead")
source("softTrimFunction.R") #or whatever you want to name this
reads<-readFastq("myreads.fq") trimmedReads<-softTrim(reads=reads,minQuality=5,firstBase=4,minLength=3) writeFastq(trimmedReads,file="trimmed.fq")

I strongly recommend reading the excellent UC Riverside HT-Sequencing Wiki cookbook and tutorial if you wish to venture into using R for NGS handling. Among other things, it will explain how to perform casting if you have Solexa scaled (base 64) fastq files. The function should respect that. http://manuals.bioinformatics.ucr.edu/home/ht-seq

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