bash: transform scaffold fasta - bash

Question

I have a fasta file with the following sequences:
>NZ_OCNF01123018.1
TACAAATACAACAAATACAAGTACACCAAGTACAAATACAAGTATCCCAAGTACAAATACAAGTA
TCCCAAGTACAAATACAAGTATTCCAAGTACAAATACAAAACCTGTTGAGCAACCTAAACCTGTTGAAC
AGCCCAAACCTGTTGAACAGCNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNAAACCTTTATCCGCACTTA
CGAGCAAATACACCAATACCGCTTTATCGGCACAGTCTGCCCAAATTGACGGATGCACCATGTTACCCAACAC
ATCAATCAACGTTTGTGGGATCACCTGAAAAAGGGCGCGGTTTGTGGTTGATG
>NZ_OCNF01123018.2
AATTGTCGTGTAAAGCCACACCAAACCCCATTATAGCCCCAAAAACACCAAAAAGGCTGCCTGAACCACATTTCAGACAG
And I want to split the all sequences in the file that contain multiple N at the site where it occurs and make two sequences out of it.
Expected solution:
>NZ_OCNF01123018.1
TACAAATACAACAAATACAAGTACACCAAGTACAAATACAAGTATCCCAAGTACAAATACAAGTA
TCCCAAGTACAAATACAAGTATTCCAAGTACAAATACAAAACCTGTTGAGCAACCTAAACCTGTTGAAC
AGCCCAAACCTGTTGAACAGC
>contig1
AAACCTTTATCCGCACTTA
CGAGCAAATACACCAATACCGCTTTATCGGCACAGTCTGCCCAAATTGACGGATGCACCATGTTACCCAACAC
ATCAATCAACGTTTGTGGGATCACCTGAAAAAGGGCGCGGTTTGTGGTTGATG
>NZ_OCNF01123018.2
AATTGTCGTGTAAAGCCACACCAAACCCCATTATAGCCCCAAAAACACCAAAAAGGCTGCCTGAACCACATTTCAGACAG
my (inelegant) approach would be this:
perl -pe 's/[N]+/\*/g' $file | perl -pe 's/\*/\n>contig1\n/g'
of course that also replaces the N of the sequence header and creates headers without a sequence. As a plus, it would be nice to number the new 'contigs' from 1 to x in case there are multiple sequences with N.
What do you suggest?

Answer 1

I'd suggest to use split instead of trying to get a regex just right, and in a script instead of a brittle and crammed "one"-liner.
use warnings;
use strict;
use feature 'say';
my $file = shift #ARGV;
die "Usage: $0 filename\n" if !$file; # also check submitted $file
my $content = do { # or: my $content = Path::Tiny::path($file)->slurp;
local $/;
open my $fh, '<', $file or die "Can't open $file: $!";
<$fh>;
};
my #f = grep { /\S/ } split /(?<!>)NN+/, $content;
say shift #f;
my $cnt;
for (#f) {
say "\n>contig", (++$cnt), ":\n$_";
}
This slurps the file into $content since NN+ can span multiple lines; Path::Tiny module can make that cleaner. The first element of the obtained array needs no >contig so it is shifted away.
The negative lookbehind (?<!...) makes the regex in split's separator pattern match NN+ only when not preceded by >, thus protecting (excluding) header lines that may start with that. If headers may contain consecutive N which are not right after > then you need to refine this.

Answer 2

I expaned your perl one-liner a bit:
cat file.fasta | \
perl -pe 's/\n//g unless /^>/; s/>/\n>/g;' | \
perl -pe 's/N+(?{$n++})/\n>contig${n}\n/g unless /^>/'
the first part is to remove newlines between bases, the second part is to replace continuous 'N'.