Dear Kaur, The XS:A tag does not tell you the strand to which the read was mapped. It tells you the strand where the exon splicing occurs. The aligner uses the donor/receptor bases (GT/AG) to determine the strand where the splicing happens. The strand which a read is mapped to is reported in the FLAG field in a SAM/BAM file and that is the information featureCounts uses to perform strand specific read summarization. I would suggest you to have a close look at your read mapping results using a genome browser like IGB so that you can have a better understanding of strandness of your reads and annotated genes. Best regards, Wei On Aug 30, 2014, at 12:44 AM, Kaur Alasoo wrote: > Dear Wei, > > Thanks for your quick reply. I think the problem might be how the strand is represented in the BAM file. I used Tophat2 to perform the alignment and it uses XS:A:+ or XS:A:- tags to specify whether the read comes from the forward or the reverse strand, respectively. When I split the reads by chromosome and then count how many of them map to each strand I get the what is expected in the annotations: > > On chromosome 11 almost all reads are on reverse strand. ENSG00000066336 is also on on the reverse strand. > $ samtools view merged.bam | awk '{if ($3 == "11") {print $0}}' | grep "XS:A:-" | wc -l > 23410 > $ samtools view merged.bam | awk '{if ($3 == "11") {print $0}}' | grep "XS:A:+" | wc -l > 5 > > On chromosome 9 almost all read are on the forward strand. ENSG00000136869 is also on the fwd strand. > $ samtools view merged.bam | awk '{if ($3 == "9") {print $0}}' | grep "XS:A:-" | wc -l > 80 > $ samtools view merged.bam | awk '{if ($3 == "9") {print $0}}' | grep "XS:A:+" | wc -l > 8628 > > Which field of the BAM files does featureCounts use to infer the strand of the read? > > Best wishes, > Kaur > > > On 29 Aug 2014, at 03:59, Wei Shi <shi at="" wehi.edu.au=""> wrote: > >> Dear Kaur, >> >> We had a close look at the reads in your supplied bam file and found that most of the reads were mapped to the positive strand. This is inconsistent with the annotation for the two genes you listed, ie. the read mapping results tell you that both genes are on the positive strand but the annotation data tell you that the two genes are on different strands. >> >> When strandSpecific is set to 1, featureCounts requires the strand of a read is the same as that of a gene before assigning the read to the gene. When strandSpecific is set to 2, featureCounts will flip the strand of each read and then try to match it to the strands of genes. Since your two genes are on different strands but the reads are almost all on the same strand, you will never get large counts (or small counts) for the two genes at the same time. >> >> This is not a problem of featureCounts. featureCounts just assigns the reads using the strand info provided in the mapping results. This is more like a problem with either the annotation or read mapping or maybe the sequencing experiment. >> >> Hope this helps. >> >> Best wishes, >> >> Wei >> >> >> >> On Aug 28, 2014, at 12:53 AM, Kaur Alasoo [guest] wrote: >> >>> Hi, >>> >>> I am trying to use featureCounts to perform strand-specifc RNA-Seq read counting with an Ensembl GTF file. However, it seems to count reads from each strand separately rather than for each gene from the strand where the gene is located. I have created a small BAM that contains reads from only two genes (one from each strand) to illustrate the problem. >>> >>> First, if I specify strandSpefic = 0 I get 97.7% of the reads aligned to two genes: >>> a = featureCounts("merged.bam", annot.ext = "Homo_sapiens.GRCh38.76.gtf", >>> strandSpecific = 0, isGTFAnnotationFile = TRUE, isPairedEnd = TRUE) >>> >>>> a$stat >>> Status counts.merged.bam >>> 1 Assigned 16179 >>> 2 Unassigned_Ambiguity 0 >>> 3 Unassigned_MultiMapping 56 >>> 4 Unassigned_NoFeatures 335 >>> >>> And the reads counts for the two genes are: >>>> a$counts[c("ENSG00000066336","ENSG00000136869"),] >>> ENSG00000066336 ENSG00000136869 >>> 11759 4419 >>> >>> However, when I try to specifiy strandSpecific = 1 or 2 I get reads from only one strand counted in either case >>> b = featureCounts("counts/merged.bam", annot.ext = "../../annotations/GRCh38/genes/Homo_sapiens.GRCh38.76.gtf", strandSpecific = 1, isGTFAnnotationFile = TRUE, isPairedEnd = TRUE) >>> >>>> b$stat >>> Status counts.merged.bam >>> 1 Assigned 5295 >>> 2 Unassigned_Ambiguity 0 >>> 3 Unassigned_MultiMapping 56 >>> 4 Unassigned_NoFeatures 11219 >>> >>>> b$counts[c("ENSG00000066336","ENSG00000136869"),] >>> ENSG00000066336 ENSG00000136869 >>> 952 4343 >>> >>> ENSG00000136869 is located on the forward strand, gets high counts with strandSpecific = 1. >>> >>> ##### >>> >>> c = featureCounts("counts/merged.bam", annot.ext = "../../annotations/GRCh38/genes/Homo_sapiens.GRCh38.76.gtf", strandSpecific = 2, isGTFAnnotationFile = TRUE, isPairedEnd = TRUE) >>> >>>> c$stat >>> Status counts.merged.bam >>> 1 Assigned 10884 >>> 2 Unassigned_Ambiguity 0 >>> 3 Unassigned_MultiMapping 56 >>> 4 Unassigned_NoFeatures 5630 >>> >>>> c$counts[c("ENSG00000066336","ENSG00000136869"),] >>> ENSG00000066336 ENSG00000136869 >>> 10807 76 >>> >>> ENSG00000066336 is located on the reverse strand, gets high counts with strandSpecific = 2. >>> >>> Furthermore, >>> a$counts[c("ENSG00000066336","ENSG00000136869"),] = c$counts[c("ENSG00000066336","ENSG00000136869"),] + b$counts[c("ENSG00000066336","ENSG00000136869"),] >>> >>> I downloaded the Ensembl 76 GTF file from here: >>> ftp://ftp.ensembl.org/pub/release-76/gtf/homo_sapiens >>> >>> The sample BAM files is available from here: >>> https://dl.dropboxusercontent.com/u/6006832/merged.bam >>> >>> Is this a bug of featureCounts or am I doing something obviously wrong? >>> >>> Kind regards, >>> Kaur Alasoo >>> PhD student at Sanger Institute >>> >>> -- output of sessionInfo(): >>> >>> R version 3.0.2 (2013-09-25) >>> Platform: x86_64-apple-darwin10.8.0 (64-bit) >>> >>> locale: >>> [1] en_GB.UTF-8/en_GB.UTF-8/en_GB.UTF-8/C/en_GB.UTF-8/en_GB.UTF-8 >>> >>> attached base packages: >>> [1] stats graphics grDevices utils datasets methods base >>> >>> other attached packages: >>> [1] Rsubread_1.12.6 BiocInstaller_1.12.1 >>> >>> loaded via a namespace (and not attached): >>> [1] tools_3.0.2 >>> >>> -- >>> Sent via the guest posting facility at bioconductor.org. >> >> ______________________________________________________________________ >> The information in this email is confidential and intended solely for the addressee. >> You must not disclose, forward, print or use it without the permission of the sender. >> ______________________________________________________________________ > > > > -- > The Wellcome Trust Sanger Institute is operated by Genome Research > Limited, a charity registered in England with number 1021457 and a > company registered in England with number 2742969, whose registered > office is 215 Euston Road, London, NW1 2BE. ______________________________________________________________________ The information in this email is confidential and intend...{{dropped:6}}