Identifying RNA-seq reads containing polyA stretch

I’m in the need to filter our RNA-seq data for reads that contain a polyA stretch (e.g. with more than 6 A’s). I need to recover those reads, not discard them.

The data is paired end and stranded. So, when dealing with paired reads in 2 files they should always be processed together, not one at a time.

Then, I would like to trim the polyA stretch and align to the reference genome only those reads that originally cointained a polyA tail.

I am aware that conventional RNA-seq is not suitable for identifying polyA sites precisely, but I am willing to try because we don’t have any other data.

Does anybody know a tool that does this? I have tried from BBMap suite for filtering and trimming, as suggested in this post. I used this command: in1=A1_R1.fastq.gz in2=A1_R2.fastq.gz outm1=A1_R1_polyA.fastq.gz outm2=A1_R2_polyA.fastq.gz literal=AAAAAA

However, I encountered some problems: 1) the output cointains 0 reads while it was supposed to contain the polyA reads with AAAAAA, and I can’t figure it out why 2) how could I indicate that I want reads with a polyA having 6 to let’s say 20 A’s?

The purpose of the analysis is to identify alternative polyAdenylation events. I am working with a parasite genome that is not well annotated.

Maybe I’m drowning in a cup of water and there is a more straightforward solution, so any help will be appreciated.

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