Bowtie2 Alignment - Usage Guide

Overview

Bowtie2 is a fast, memory-efficient short-read aligner whose defining choice is end-to-end (the whole read must align) vs local (read ends may be soft-clipped) mode. It is the de-facto aligner for ChIP-seq, ATAC-seq, and CUT&RUN, and it is the engine Bismark wraps for bisulfite data. For peak assays the alignment itself matters less than the mode, the sensitivity preset, and the fragment-geometry flags (--no-mixed, --no-discordant, --dovetail, -X) that determine the fragment coordinates the peak caller consumes -- this skill emphasizes those.

Prerequisites

conda install -c bioconda bowtie2 samtools

• A Bowtie2 index (built with bowtie2-build) or a prebuilt index; pass the basename to -x.
• For ChIP/ATAC, decide whether reads are trimmed (end-to-end) or carry adapter read-through (local) before choosing the mode.

Quick Start

Tell your AI agent what you want to do: - "Align my ChIP-seq reads with Bowtie2 and filter multimappers" - "Build a Bowtie2 index for my reference genome" - "Align ATAC-seq reads with the right fragment and soft-clipping settings" - "Choose end-to-end vs local mode for my data and explain why" - "Report up to 5 alignments per read for a repetitive region"

Example Prompts

ChIP-seq

"Align my paired-end ChIP-seq reads with high sensitivity, keep only concordant proper pairs, and filter to MAPQ >= 30 before peak calling."

ATAC-seq

"Align ATAC-seq reads allowing fragments up to 2 kb and soft-clipping adapter read-through, and tell me which flags handle the short dovetailed fragments."

Mode choice

"My reads have adapter contamination at the 3' end and a low alignment rate in end-to-end mode -- should I trim or switch to local mode?"

Multi-mapping

"Align reads and report up to 5 alignments per read for a repetitive region, and explain why MAPQ is unreliable in -k mode."

What the Agent Will Do

1. Build the index with bowtie2-build if needed, and pass the basename (not a filename) to -x.
2. Choose end-to-end vs local mode from the data (trimmed clean DNA vs adapter-contaminated/ATAC) and a sensitivity preset.
3. Set the fragment-geometry flags for the assay: --no-mixed --no-discordant for ChIP, plus --local --dovetail -X 2000 for ATAC.
4. Filter to a Bowtie2-appropriate MAPQ (-q 30, not a BWA-style -q 60) and a flag mask (-F 1804) before peak calling.
5. Add read groups (--rg-id/--rg) and stream to a sorted, indexed BAM.
6. Route peak calling and the ATAC Tn5 cut-site shift to the chip-seq / atac-seq categories, and the QC gate to alignment-files/bam-statistics.

When to Use Bowtie2 vs bwa-mem2

Tips

• Choose the mode from the data: end-to-end for clean trimmed DNA, --local for adapter read-through or ATAC.
• Never copy a BWA MAPQ >= 60 filter to Bowtie2 output -- the scale caps at 42 (end-to-end) / 44 (local); use -q 30.
• For ATAC, add --dovetail and -X 2000 or real short-fragment pairs get flagged discordant and dropped.
• Pass the index basename to -x, not a .bt2 file.
• Bowtie2 writes its alignment summary to stderr -- capture it (2> align.log) for QC and MultiQC aggregation.
• The Tn5 +4/-5 cut-site shift for ATAC is a downstream signal-track transform, not an alignment flag; route it to the atac-seq category.

Related Skills

• bwa-alignment - DNA variant-calling alignment with bwa-mem2 (ALT/decoy-aware)
• star-alignment - RNA splice-aware alignment (when reads cross junctions)
• read-qc/fastp-workflow - Trim adapters before end-to-end alignment
• alignment-files/duplicate-handling - Mark/remove duplicates after alignment
• alignment-files/bam-statistics - flagstat/idxstats QC gate; the cross-tool MAPQ scale via sam-bam-basics
• chip-seq/peak-calling - Call peaks from ChIP/CUT&RUN BAMs
• atac-seq/atac-peak-calling - ATAC peak calling and the Tn5 cut-site shift
• methylation-analysis/bismark-alignment - Bisulfite alignment (wraps Bowtie2)