I have Bed file containing start and end of a sequence, and I need to convert it to fasta format, any recommendations?

  • 4
    $\begingroup$ Please edit your question and give us more details. Bed files define regions, do you have a fasta file of the target regions? Is this for a genome? Do you have that genome? $\endgroup$
    – terdon
    Nov 5, 2018 at 11:28

3 Answers 3


bedtools getfasta can do this. See documentation.


You could start with a folder of samtools faidx-indexed FASTA files for your genome of interest:

$ mkdir /genomes/hg38 && cd /genomes/hg38
$ wget ftp://hgdownload.cse.ucsc.edu/goldenPath/hg38/chromosomes/*.fa.gz
$ for fn in `ls *.fa.gz`; do gunzip $fn; done
$ for fn in `ls *.fa`; do samtools faidx $fn; done

Then use that folder with the following script:

#!/usr/bin/env perl

use strict;
use warnings;
use Getopt::Long;

# bed2faidxsta.pl
# --
# Reads BED data from standard input, writes FASTA to standard output.
# Dependent on samtools and indexed FASTA data files located in $fastaDir 
# variable. Set --fastaDir=dir to set custom directory containing a source 
# of per-build, bgzip-compressed FASTA and associated index (fa.gz.fai) 
# files, or leave unset to use data in current working directory. Use the
# --fastaIsUncompressed option if the FASTA files are not compressed.

# test if samtools is available
`samtools --version` || die "Error: The samtools application is required to run this script. Try 'module add samtools' or install a local copy of samtools.\n";

# default FASTA input is current working directory
my $fastaDir = `pwd`; chomp $fastaDir;
# default is to assume input coordinates use zero-based index scheme
my $oneBased;
# default is to leave IDs alone
my $useIDPrefixAsStrand;
# default is to assume FASTA files are bgzip-compressed
my $fastaIsUncompressed;

GetOptions ('fastaDir=s' => \$fastaDir, 'oneBased' => \$oneBased, 'useIDPrefixAsStrand' => \$useIDPrefixAsStrand, 'fastaIsUncompressed' => \$fastaIsUncompressed);

if (! -d $fastaDir) { die "Error: FASTA directory does not exist\n"; }

while (<STDIN>) {
    my ($chr, $start, $stop, $id, $score, $strand) = split("\t", $_);
if (!defined($chr) || !defined($start) || !defined($stop)) { die "Error: No chromosome name, start or stop position defined\n"; }
    if (!defined($id)) { $id = "."; }
if (!defined($score)) { $score = "."; }
if (!defined($strand)) { $strand = "+"; } else { $strand = substr($strand, 0, 1); }
# adjust coordinates to one-based index, if necessary
my ($queryChr, $queryStart, $queryStop) = ($chr, $start, $stop);
if (!$oneBased) {
# adjust strand if required
if ($useIDPrefixAsStrand) {
        $strand = substr($id, 0, 1);
    # lookup
    my $queryFn = "$fastaDir/$chr.fa.gz";
if ($fastaIsUncompressed) {
        $queryFn = "$fastaDir/$chr.fa";
my $queryKey = "$queryChr:$queryStart-$queryStop";
my $queryResult = `samtools faidx $queryFn $queryKey`; chomp $queryResult; 
# linearize result
my @lines = split("\n", $queryResult);
    my @seqs = @lines[1..(scalar @lines - 1)];
    my $seq = join("", @seqs);
# handle reverse-stranded elements
if ($strand eq "-") {
        $seq = rc_sequence($seq);
    # print to standard output
    my $header = ">".join(":",($chr, $start, $stop, $id, $score, $strand));
print STDOUT $header."\n".$seq."\n";

sub rc_sequence {
    my $seq = shift @_;
my $reverse_complement = reverse($seq);
$reverse_complement =~ tr/ACGTacgt/TGCAtgca/;
    return $reverse_complement;


$ perl bed2faidxsta.pl < in.bed > out.fa

This approach can be useful with stranded input, nonstandard genomes, or just to do things with a bit more knowledge of inputs.

  • $\begingroup$ Or, since you already have samtools, the poster can use samtools faidx with a file of regions. It won't be hard to get a bedfile into the right format: htslib.org/doc/samtools.html $\endgroup$
    – swbarnes2
    Nov 5, 2018 at 18:37

UCSC 'kent' command line tool:

faToTwoBit -bed=yourCoordinates.bed genomeSequence.2bit output.fa

Binary for this command and others available at: UCSC kent command line tools

  • $\begingroup$ I think you mean twoBitToFa. Thanks anyway for pointing me in that direction. $\endgroup$ Mar 10, 2019 at 10:43

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