Prophage Tracer

Prophage Tracer: Precisely tracing prophages in prokaryotic genomes using overlapping split-read alignment. Based on our analysis, in order to detect prophages with low excision rate, 100–1000× sequencing depth for a genome is recommended. At this range of sequencing depth, Prophage Tracer can detect the hidden prophages with excision rates (attB/gyrB) > $10^-3$ and/or replication (attP/gyrB) > $10^-3$ in host genomes.

Requirement

System and software requirements

1. Linux (Tested in CentOS 6.8 and CentOS Linux release 7.8.2003 (GNU Awk 4.0.2))
2. blastn: 2.6.0+, Download ncbi-blast-2.6.0+-x64-linux.tar.gz for linux system.
3. bwa 0.6
4. sambamba 0.8.0
5. samtools 1.10

Other softwares may be useful for data pre-processing steps

1. Shovill 1.1.0 for assembling genomes. It is useful for detecting prophages assembled into their own separate contigs in contig-level genomes. In this case, the average sequencing depth of prophage-derived contigs is usually but not necessarily significantly higher than other contigs. The depth is written into the name of each contig in the output of Shovill.
2. Trimmomatic 0.39 for removing low-quality regions and adapters in reads.

Installation

1. Just download the shell scripts prophage_tracer.sh and prophage_tracer.sh to your working directory
2. Install required softwares through Conda

first install conda

wget -c https://repo.continuum.io/miniconda/Miniconda2-latest-Linux-x86_64.sh
bash Miniconda2-latest-Linux-x86_64.sh

Changing your environment variables temporarily to use conda

export PATH=~/miniconda2/bin:$PATH

conda install -c bioconda bwa
conda install -c bioconda sambamba
conda install -c bioconda samtools

Run Prophage Tracer

1. Download the prophage_tracer.sh and put it in your working path

2. Prepare aligned reads in SAM file

Assume that you have a sequenced bacterium (strain1) genome in FASTA format reference_genome_strain1.fasta, and paried reads in FASTQ format 1.fastq.gz and 2.fastq.gz

• Align reads to the reference genome

bwa index reference_genome_strain1.fasta -p strain1
bwa mem strain1 1.fastq.gz 2.fastq.gz >strain1.sam

• Remove duplicate (this step is not necessary but will improve the output)

samtools view -S -b strain1.sam -o strain1.bam
sambamba markdup -r strain1.bam strain1.rmdup.bam
samtools view strain1.rmdup.bam -o strain1.rmdup.sam 

3. Run prophage_tracer.sh

bash prophage_tracer.sh -m strain1.rmdup.sam -r reference_genome_strain1.fasta -p strain1

Usage

usage:   prophage_tracer [options] -m <in.sam> -r <in.fasta> -p <prefix>

options:
     -m  FILE    a full SAM file (required)
     -r  FILE    a reference genome sequence (required)
     -p  STRING  prefix of output files (required; usually a strain name or a sample name)
     -x  INT     maximal size of a prophage (default: 150000)
     -n  INT     minimal size of a prophage (default: 5000)
     -a  INT     minimal length of attchment site (default: > 2)
     -t  INT     number of threads used for BlastN (default: 1)
     -s  INT     minimal event of split reads required for supporting a prophage candidate (default: 1)
     -d  INT     minimal event of discordant read pairs required for supporting a prophage candidat (default: 1)

Typical output

• Find result in strain1.prophage.out

prophage_candidate	contig	attL_start	attL_end	attR_start	attR_end	prophage_size	SR_evidence_attB	SR_evidence_attP	DRP_evidence_attB	DRP_evidence_attP
candidate_1	contig00007=::=contig00014	209162	209236	2365	2439	16770	0	4	1	2
candidate_2	contig00001	1064123	1064145	1100156	1100178	36033	0	1	0	0
candidate_3	=contig00003::=contig00004	1700	1764	46895	46959	48658	2	28	2	24

Explanation

1. If a single contig was given in the contig column, it means an intact predicted prophage is in this contig.
2. "::" indicates a predicted prophage is separated into two or more contigs. "=" indicates that the 5' end or 3' end of a contig contains a part of a prophage. "=contig00003" indicates the 5' end of contig00003 while "contig00003=" indicates the 3' end of contig00003.
3. "SR_evidence_attB/attP" indicate split read counts support attB or attP of the predicted prophage. "DRP_evidence_attB/attP" indicate discordant read pair counts support attB or attP of the predicted prophage.
4. Prophage Phm2 (candidate_2) locates on contig00001 (1064123-1100178) with attL (1064123-1064145) and attR (1100156-1100178). Prophage Phm1 (candidate_1) is separated into two or more contigs and consists at least 3' end of contig00007 (att site: 209162-209236 on contig00007) and 5' end of contig00014 (att site: 2365-2439 on contig00014). Prophage Phm3 (candidate_3) is seperated into two or more contigs and consist at least 5' end of contig00003 (att site: 1700-1764 on contig00003) and 5' end of contig00014 (att site: 46895-46959 on contig00014).

Notes

1. prophage_tracer.sh is used for chromosome-level genomes. prophage_tracer_WGS.sh can be used for chromosome-level and contig-level genomes. However, using prophage_tracer_WGS.sh for analysis of chromosome-level genomes would be slow.
2. If a prophage is separated into two or more contigs, the predicted prophage size might be smaller than the true size.
3. If you have generated a SAM file using samtools, our script can be equally used to predict prophages on Windows 10 with Git Bash and blastn (ncbi-blast-2.6.0+-win64.exe) installed. For installing Git Bash and blastn in Windows 10, please refer to https://git-scm.com/downloads and https://ftp.ncbi.nlm.nih.gov/blast/executables/blast+/2.6.0/.

Using generate_DNA.sh for generating simulated genomes resulted from prophage excision

Install seqkit first

conda install -c bioconda seqkit

Download generate_DNA.sh and random_DNA.py

Run script (default: simulating 20 genomes; one prophage in each genome)

bash generate_DNA.sh

Coming updates

1. Adding a parameter of blastn to set whether mismatch were allowed in the att sites (-penalty).
2. To make the script to be able to analyze single read sequencing data.

Copyright

Kaihao Tang, khtang@scsio.ac.cn; Xiaoxue Wang, xxwang@scsio.ac.cn; Marine Biofilm Lab; SCSIO, Chinese Academy of Sciences