Task 2 lab journal (MSUD)

Multiple sequence alignments

Dataset creation

The datasets were created from the Blast output.

For creating datasets with low, high and whole range sequence identity, the following Python script was used:

<source lang="python"> Use blast output to create datasets with different sequence identities.

Call with: python create_dataset.py <query fasta file> <blast xml output> <database fasta file>

@author: Laura Schiller

import sys from Bio import SeqIO, pairwise2 from Bio.Blast import NCBIXML from Bio.SubsMat import MatrixInfo

def get_sequences(query, blast_xml, db_path, out_file):

   
   Fetch full length sequences for a BLAST result and write them in a FASTA file.
   
   @param query:     fasta file with query sequence.
   @param blast_xml: xml file with BLAST search result.
   @param db_path:   path of db fasta file.
   @param out_file:  file to store the sequences.
   @return:          name of the file where the sequences are stored.
   
   
   print("get all sequences for blast result in %s" % blast_xml)
   query_sequence = SeqIO.read(query, "fasta")
   hit_seqs = [query_sequence]
   
   blast_output = open(blast_xml)
   blast_result = NCBIXML.read(blast_output)
   blast_output.close()
   
   hit_list = []
   for alignment in blast_result.alignments:
       hit_list.append(alignment.title.split(" ")[1]) # the id of the sequence    
   
   # get all blast hits
   seqs_db = SeqIO.parse(db_path, "fasta")
   counter = 0
   number = len(blast_result.alignments)
   for seq in seqs_db:
       if seq.id in hit_list:
           hit_seqs.append(seq)
           counter = counter + 1
           if (counter % 100) == 0:
               print("%d of %d sequences found" % (counter, number))
           if counter == number:
               break
   print("%d of %d sequences found" % (counter, len(blast_result.alignments)))        
   
   SeqIO.write(hit_seqs, out_file, "fasta")
   
   print("sequences saved in %s" % out_file)
   return out_file

def filter_seqs(seq_file, name):

   
   Filter sequences according to sequence identity limits.
   
   @param seq_file: fasta file with sequences.
   @param name:     string used for output file names.
   
   
   sequences = SeqIO.parse(seq_file, "fasta")
   seqs = []
   for seq in sequences:
       seqs.append(seq)
   query = seqs.pop(0) # always keep query (first sequence)
   # lists for low / high / whole range sequence identity
   filtered = [[query], [query], [query]]
   
   # identify hits with pdb structures -> these are preferentially taken
   hits_pdb = [hit for hit in seqs if (hit.id.split("|")[0] == "pdb")]
   seqs = [seq for seq in seqs if not (seq.id in [pdb_seq.id for pdb_seq in hits_pdb])]
   hits_pdb.extend(seqs) # now pdb hits are at the beginning
   
   print("filter sequences")
   for seq in hits_pdb:           
       try: 
           ident = identity(query, seq) 
       except KeyError: # raises if there is a non amino acid letter
           continue
       if (len(filtered[0]) < 10):
           keep = True            
           for seq2 in filtered[0]:
               try: 
                   ident2 = identity(seq, seq2) 
               except KeyError:
                   keep = False
                   continue
               if ident2 >= 0.3:
                   keep = False
                   break
           if keep:
               filtered[0].append(seq)
       if (len(filtered[1]) < 10) and (ident > 0.6):
           filtered[1].append(seq)
       if (len(filtered[2]) < 8) and (ident >= 0.3) and (ident <= 0.6):
           filtered[2].append(seq)
       if (len(filtered[0]) == 10) and (len(filtered[1]) == 10) and (len(filtered[2]) == 8):
           break
   
   # for whole range take a part of low and a part of high sequence identity
   # plus the rest middle sequence identity
   filtered[2].extend(filtered[0][1:min(len(filtered[0]), 7)])
   filtered[2].extend(filtered[1][1:min(len(filtered[1]), 7)])
   
   SeqIO.write(filtered[0], name + "_low_seq_ident.fasta", "fasta")
   SeqIO.write(filtered[1], name + "_high_seq_ident.fasta", "fasta")
   SeqIO.write(filtered[2], name + "_whole_range_seq_ident.fasta", "fasta")
   
   print("sequences with low / high / whole range sequence identity saved in:")
   print(name + "_low_seq_ident.fasta")
   print(name + "_high_seq_ident.fasta")
   print(name + "_whole_range_seq_ident.fasta") 

def identity(seq1, seq2):

   #pairwise alignment
   matrix = MatrixInfo.blosum62
   gap_open = -10
   gap_extend = -0.5     
   alignment = pairwise2.align.globalds(seq1, seq2, matrix, gap_open, gap_extend)
   seq1_aligned = alignment[0][0]
   seq2_aligned = alignment[0][1]

   #sequence identity
   ident = sum(c1 == c2 for c1, c2 in zip(seq1_aligned, seq2_aligned))
   ref_length = (len(seq1) + len(seq2)) / 2 # mean length
   return float(ident) / ref_length

if __name__ == '__main__':

   namestring = sys.argv[1].split("/")[-1].split(".")[0] # used as beginning of the output files
   print("-----------------------------------------------------------")
   all_seqs = get_sequences(sys.argv[1], sys.argv[2], sys.argv[3], namestring + "_all_sequences.fasta")
   filter_seqs(all_seqs, namestring)

</source>

Calling MSA programs

Call of T-Coffee:

#!/bin/bash

proteins=( BCKDHA BCKDHB DBT DLD )
identities=( low high whole_range )

for protein in ${proteins[*]}
do
	for identity in ${identities[*]}
	do
		t_coffee -output fasta -infile ${protein}_${identity}_seq_ident.fasta -outfile ${protein}_${identity}_seq_ident_tcoffee.fasta 
	done
done

Muscle:

#!/bin/bash

proteins=( BCKDHA BCKDHB DBT DLD )
identities=( low high whole_range )

for protein in ${proteins[*]}
do
	for identity in ${identities[*]}
	do
		muscle -in ${protein}_${identity}_seq_ident.fasta -out ${protein}_${identity}_seq_ident_muscle.fasta 
	done
done

Mafft:

#!/bin/bash

proteins=( BCKDHA BCKDHB DBT DLD )
identities=( low high whole_range )

for protein in ${proteins[*]}
do
	for identity in ${identities[*]}
	do
		mafft ${protein}_${identity}_seq_ident.fasta > ${protein}_${identity}_seq_ident_mafft.fasta  
	done
done