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Summary

Bio::Tools::SeqPattern::Backtranslate

Package variables

Included modules

Inherit

Bio::Root::Root

Synopsis

No synopsis!

Description

This module should not be used directly. It provides helper methods to
Bio::Tools::SeqPattern to reverse translate protein patterns.

Methods

Methods description

None available.

Methods code

sub _reverse_translate_motif {

   # Main subroutine. It takes a Profam-like motif and returns its
   # reverse translation using degenerate codons.

   # Steps:
   # 1. Tokenize, then parse tokens.
   # 2. Reverse translate each token type.
   # 3. Join tokens in original order. Return the resulting string.

   my $motif = shift;

   $motif =~ s/\./X/g;
   $motif = uc $motif;

   ### 1. Tokenize, parse the motif.
   my ( $ordered, $classified ) = _parse_motif($motif);

   ### 2. Reverse translate each token type.
   # Reverse translate the plain (unambiguous) tokens.
   my $ct = Bio::Tools::CodonTable->new;
   foreach my $seq ( @{ $classified->{plain} } ) {
      my $seqO
          = Bio::Seq->new( -seq => $$seq, -alphabet => 'protein' );
      $$seq = $ct->reverse_translate_all($seqO);
   }

   # Reverse translate the ambiguous tokens.
   foreach my $token ( @{ $classified->{ambiguous} } ) {
      my ($aas) = $$token =~ m(([A-Za-z\.]+));
      my @codons_to_contract;

      foreach my $residue ( split '', $aas ) {
         push @codons_to_contract, $ct->revtranslate($residue);
      }

      my $ambiguous_codon = _contract_codons(@codons_to_contract);
      $$token             = $ambiguous_codon;
   }

   # Reverse translate the negated residues.
   foreach my $token ( @{ $classified->{negated} } ) {
      my ($aas) = $$token =~ m(([A-Za-z\.]+));
      my $ambiguous_codon = _negated_aas_to_codon($aas);
      $$token = $ambiguous_codon;
   }

   ### 3. Join the profile back from its tokens.
   return join '', map {$$_} @{$ordered};

}

sub _parse_motif {

   # Profam-like motif parser. It takes the pattern as a string, and
   # returns two data structures that contain the tokens, organized
   # by order of appearance in the pattern (first return value) and by
   # category (second return value).

   my $motif = shift;
   my $parser  = _tokenize_motif($motif);
   my ( %tokens, @tokens );

   while ( my $token = $parser->() ) {
      croak ("Unknown syntax token: <", $token->[1], ">")
          if ( $token->[0] eq 'UNKNOWN' );
      push @{ $tokens{ $token->[0] } },\$ token->[1];
      push @tokens,\$ token->[1];
   }
   return (\@ tokens,\% tokens );

}

sub _tokenize_motif {

   # Return a tokenizer iterator that sequentially recognizes and
   # returns each token in the input pattern.
   # Examples of each token type:

   # ambiguous: a position with more than one possible residue.
   #     eg. [ALEP]
   # negated: a position in which some residues are excluded.
   #     eg. [^WY]
   # plain: a common sequence of residues. One position, one residue.
   #     eg. MAAEIK
   # open_par, close_par: tags surrounding a motif that is repeated
   #     a certain number of times.
   #     eg. (...){3}

   my $target = shift;
   return sub {
      return [ 'ambiguous', $1 ]
          if $target =~ /\G (\[[A-Za-z\.]+\])     /gcx;
      return [ 'negated',   $1 ]
          if $target =~ /\G (\[\^[A-Za-z\.]+\])   /gcx;
      return [ 'plain',     $1 ]
          if $target =~ /\G ([A-Za-z\.]+)         /gcx;
      return [ 'open_par',  $1 ]
          if $target =~ /\G (\()                  /gcx;
      return [ 'close_par', $1 ]
          if $target =~ /\G (\)[\{\d+[,\d+]*\}]*) /gcx;
      return [ 'UNKNOWN',   $1 ]
          if $target =~ /\G (.)                   /gcx;
      return;
   };

}

sub _contract_codons {

   # Take a list of codons, return an ambiguous codon.
   my @codons = map { uc $_ } @_;

   my @by_letter = ( [], [], [], );
   my $ambiguous_codon;
   foreach my $codon (@codons) {
      my @letters = split '', $codon;
      for my $i ( 0 .. 2 ) {
         push @{ $by_letter[$i] }, $letters[$i];
      }
   }
   for my $i ( 0 .. 2 ) {
      $ambiguous_codon
          .= _convert( 'dna', _uniq_string( @{ $by_letter[$i] } ) );
   }
   return $ambiguous_codon;

}

sub _expand_codon {

   # Given a degenerate codon, return a list with all its
   # constituents. Takes a three-letter string (codon) as
   # input, returns a list with three-letter scalars.

   my $codon = shift;
   die "Wrong codon length!\n" if length $codon != 3;


   my ( @codons, @return_bases );
   my @orig_bases = split '', $codon;

   for my $i ( 0 .. 2 ) {

      # from each redundant base, create a list with all their
      # components (e.g., N -> (A, C, G, T) );
      my @components = split '', _convert('dna', $orig_bases[$i] );
      $orig_bases[$i] = [@components];
   }

   # Combine all the bases of each of the three positions of the
   # codons, and build the return list.
   for my $i ( @{ $orig_bases[0] } ) {
      for my $j ( @{ $orig_bases[1] } ) {
         for my $k ( @{ $orig_bases[2] } ) {
            push @return_bases, $i . $j . $k;
         }
      }
   }
   return @return_bases;
}

{
   my %convert;

}

sub _convert {

      # Interconvert between redundant and non-redundant protein and
      # dna alphabets. Takes an alphabet (protein or dna) and a string
      # with the letter, and returns its equivalent in
      # redundant/non-redundant alphabet. Example ACTG -> N.

      my ($alphabet, $letter) = @_;
      unless (
         $alphabet and $alphabet =~ /^dna$|^protein$/i
         and $letter and length $letter <= 4
      ) { croak "Wrong arguments!\n"; }

      unless (%convert) {
         %convert = (
            'dna' => {
               qw(N ACGT B CGT D AGT H ACT V ACG K GT
                   M AC R AG S CG W AT Y CT A A C C T T G G)
            },
            'protein' => {
               '.' => 'ACDEFGHIJKLMNOPQRSTUVWY',
               X   => 'ACDEFGHIJKLMNOPQRSTUVWY',
               Z   => 'QE',
               B   => 'ND',
            },
         );

         # Make %convert hash key/value agnostic.
         foreach my $alphabet ( keys %convert ) {
            map { $convert{$alphabet}->{ $convert{$alphabet}{$_} } = $_ }
                keys %{ $convert{$alphabet} };
         }
      }

      return $convert{$alphabet}{$letter};
   }

}

sub _uniq_string {

   # Takes a list of letters and returns an alphabetically sorted
   # list with unique elements.

   my @letters = @_;
   return join '', sort { $a cmp $b } uniq @letters;
}

{
   my ( @codon_library, $ct );

}

sub _negated_aas_to_codon {

      # Given a string of residues, returns a degenerate codon that will
      # not be translated into any of them, while maximizing degeneracy
      # (ie, it tries to also translate into as many residues as possible).

      # This functionality is required for reverse translating profiles
      # that contain negative patterns: [^X]. This means that the current
      # position should not contain aminoacid X, but can have any of the
      # others. The reverse translated nucleotide sequence should
      # reflect this.

      # Approach: construct a list of all possible codons, incluiding all
      # degenerate bases. This is an array of 15x15x15 = 3375 elements.
      # Order them by descendent "degeneracy".
      # Return the first one whose expansion in 4-lettered codons
      # doesn't contain a codon that translates into any of the
      # non-wanted residues.
      
      # * Since this takes some time, I presorted them and saved them.
      # Reading them from a file takes a fraction of the time that it taes
      # to re-sort them every time the application is launched.

      my $aas_to_avoid = shift;

      # Initialize reusable variables if it's the first time the sub
      # is called.
      unless (@codon_library) {
         while (<DATA>) { chomp; push @codon_library, split ' ', $_ }
      }
      unless ($ct) { $ct = Bio::Tools::CodonTable->new; }

      # Reverse translate the unwanted aminoacids to unwanted codons.
      my @unwanted_codons;
      foreach my $aa ( split '', $aas_to_avoid ) {
         push @unwanted_codons, $ct->revtranslate($aa);
      }

      foreach my $degenerate_codon (@codon_library) {
         my @codons  = _expand_codon($degenerate_codon);
         my $success = 1;

         foreach my $unwanted (@unwanted_codons) {
            if ( grep { uc $unwanted eq $_ } @codons ) {
               $success = 0;
            }
         }

         if ($success) { return $degenerate_codon }
      }
   }

}

1;

}

General documentation

Copyright 2009 Bruno Vecchi, all rights reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.