# Simplifier: a web tool to eliminate redundant NGS contigs

Rommel Thiago Jucá Ramos<fnref n="1" />, Adriana Ribeiro Carneiro<fnref n="1" />, Vasco Azevedo², Maria Paula Schneider<fnref n="1" />, Debmalya Barh³* & Artur Silva<fnref n="1" />

Received August 22, 2012; Accepted August 28, 2012; Published October 13, 2012

**Abstract:**

Modern genomic sequencing technologies produce a large amount of data with reduced cost per base; however, this data consists of short reads. This reduction in the size of the reads, compared to those obtained with previous methodologies, presents new challenges, including a need for efficient algorithms for the assembly of genomes from short reads and for resolving repetitions. Additionally after *abinitio* assembly, curation of the hundreds or thousands of contigs generated by assemblers demands considerable time and computational resources. We developed Simplifier, a stand-alone software that selectively eliminates redundant sequences from the collection of contigs generated by *ab initio* assembly of genomes. Application of Simplifier to data generated by assembly of the genome of *Corynebacterium pseudotuberculosis* strain 258 reduced the number of contigs generated by *ab initio* methods from 8,004 to 5,272, a reduction of 34.14%; in addition, N50 increased from 1 kb to 1.5 kb. Processing the contigs of *Escherichia coli* DH10B with Simplifier reduced the mate-paired library 17.47% and the fragment library 23.91%. Simplifier removed redundant sequences from datasets produced by assemblers, thereby reducing the effort required for finalization of genome assembly in tests with data from Prokaryotic organisms.

**Availability:** Simplifier is available at http://www.genoma.ufpa.br/rramos/softwares/simplifier.xhtml. It requires Sun jdk 6 or higher.

**Key words:** NGS sequencing, *ab initio* assembly of genomes, redundant sequences

**Background:**

NGS platforms utilize various different strategies for sequencing genomes and are capable of generating a large quantity of data with great accuracy and reduction in time necessary for sequencing and cost per base [1, 2]. However, they require considerable computational infrastructure for data processing [3]. The reduced size of the sequences that are initially generated presents challenges for the genome assembly process, including resolution of repetitive regions and a need to

process extremely large numbers of reads, several magnitudes greater than produced by previous technologies [4].

Among strategies for obtaining complete genomes via NGS, there are hybrid approaches, involving the use of various assembly algorithms and combinations of data from various sequencing platforms to take advantage of complementarity among data sets. The redundancy of reads gives greater coverage of sequencing, which favors the generation of contigs that can represent regions in common or new regions of the

<footnote n="1">¹Instituto de Ciências Biológicas, Universidade Federal do Pará, Belém, PA, Brazil; ²Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil; ³Centre for Genomics and Applied Gene Technology, Institute of Integrative Omics and Applied Biotechnology (IIOAB), Nonakuri, Purba Medinipur, WB-721172, India; Debmalya Barh - Email: dr.barh@gmail.com; Tel/Fax: +91 944 955 0032; *Corresponding author</footnote>
genome, depending on the methodologies that are chosen [5]. The large number of contigs generated by these approaches demand considerable computational and human resources for their analysis, mainly for the identification of assembly errors and for elimination of the few wrong bases at the ends of contigs, which prevents overlapping extension due to mismatches [6-8].

To this end, we developed Simplifier; a stand-alone application that eliminates redundant sequences from groups of contigs generated by *ab initio* methodology, facilitating analysis of the data, reducing the time needed for the finalization and curation of the genome assemblies.

<figure><img src="image_1.png" /><figcaption>**Figure 1:** Interface of analysis of contigs of G4ALL. Superposition demonstrating that the last three bases of contig_2648 (white) are different from those of contig NODE_185 (black) and the reference. After elimination of these bases, there is redundancy between the sequences.</figcaption></figure>

<figure><img src="image_2.png" /><figcaption>**Figure 2:** Criterion of elimination of redundancy of the contigs. Each contig of a group is compared with the others, considering the following possibilities: the – Contig completely redundant; B- Contig that when trimmed at the 5' end is redundant; C- Contig that when trimmed at the 3' end is redundant; D- Contig that when trimmed at both ends are redundant.</figcaption></figure>

**Software:**

**Input and output:**

**Data**

The data that we tested with this software were obtained by sequencing *Escherichia coli* DH10B http://www.ncbi.nlm.nih.gov/sra/SRX000353 with the SOLiD Version 3 sequencer; it provided a 50x50 mate-paired library with 28,627,096 reads and 37,365,488 (50 bp) fragments from http://solidsoftwaretools.com/gf/project/. We also processed genome data of *Corynebacterium pseudotuberculosis* strain 258 (Cp258), accession number CP003540.1, using the platform SOLiD Version 3 Plus, which provided a library of 50 bp fragments.

**Sequence assembly pipeline**

The sequencing reads were submitted to an assembly pipeline, which began with the software SAET (Life Technologies) for the

correction of sequencing errors, followed by Quality Assessment [8] for analysis of quality and application of the Phred 20 filter to the reads, to reduce the probability of assembly errors [9].

The processed data was then submitted to *ab initio* assembly, through the Bruijn approach, using the software Velvet [10] and overlap-layout-consensus with Edena [6]. The contigs obtained were aligned against the reference genome and visualized with the software Graphical Contig Analyser for All Sequencing Platforms – G4ALL (unpublished data), which identifies erroneous bases at the ends of the sequences (Figure 1).

**Simplifier**

Simplifier was implemented in JAVA (http://www.java.sun.com) utilizing the Swing library. The parallel processing was carried out with the use of threads, to improve the performance of the software. Simplifier receives multi fasta file containing contigs as input. Another file with relevant statistical informations such as N50 longest and shortest contig and number of bases. In the local version, these are entered with the command: java -jar Simplifier.jar -i <input file> -c <number of bases to be trimmed> -o <output file>. It then analyzes the information on how many bases at the ends of the contigs can be disregarded for the identification of redundant sequences. And finally it produces output file which contains contigs without redundancies from multifasta file produced by assemblers.
Steps for reducing the number of contigs

When processing the data with Simplifier, the size of each
sequence is calculated and stored in a list in decreasing order of
contig size. Beginning with the smallest contig in the list, the
program searches for larger ones can contain it, and thus, each
contig is compared with all the others in the list. The redundant
sequences recognized during the process are not considered in
the subsequent comparisons, which reduces processing time.

The operator stipulates the maximum number of bases to be
trimmed at the ends of the contigs; Simplifier trims from 0 up to
the maximum number of bases defined. The search for
redundancy is carried out in the following order: perfect
matches, which are identified by the use of the sequences as a
key of a hash table, trimming the 5' end, trimming the 3' end,
and finally trimming both ends (Figure 2). After identifying
redundancy, the sequence is saved to be used in the generation
of the results file in multifasta format.

<figure><img src="image_3.png" /></figure>

**Figure 3:** Relation between the frequency of contigs of Escherichia coli DH10B and its size range. The X-axis shows the size of the contigs; the Y-axis indicates the number of contigs in each range/library for the fragment and mate-paired libraries, with and without the use of Simplifier.

Utility:

After applying a quality filter (phred 20) to the DH10B data,
there were still 6,268,564 and 7,900,511 reads for the F3 and R3
mate-paired libraries, respectively, and 26,825,018 for the
fragment library. In the assembly of the genomes, summing the
contigs generated by Velvet and Edena, utilizing k-mer 29,
resulted in 17,149 contigs for the mate-paired library, and 24,344
contigs for the fragments, utilizing k-mer 29 and 31, for Velvet
and Edena, respectively. Using G4ALL, we observed
mismatches in the bases at the ends of various contigs, mainly
in the 3' region. Consequently, as a parameter of Simplifier, we
defined three as the maximum number of bases to be trimmed
at the ends of the sequences, in order to identify redundancies.

After processing contigs with Simplifier, there was a reduction
of 17.47% (2,996 contigs) for mate-paired and 23.91% (5,821
contigs) for fragment libraries **Table 1** (see **supplementary
material**). In the two sets of data, few contigs less than or equal
to 100 bp showed redundancy; most of the contigs eliminated
by Simplifier were the size range of 101 to 499 bp; no contigs

longer than 3 kb were eliminated **Figure 3 and Table 2 (see supplementary material)**, resulting in an increase in N50 values (**Table 1**).

We ran BLAST against the NCBI non-redundant databank
utilizing a server with two Intel Xeon E5530 Quad Core 2.40
GHz processors and 24 Gb RAM) for identifying the products
contained in the contigs; Simplifier reduced processing time up
to 17 h for the fragments library, with about one million less
hits compared to the original group that contained redundant
contigs (Table 1). Simplifier used 62 minutes, 35 seconds
processing time for the fragment library data (24,344 contigs)
and 35 minutes, 41 seconds for the mate-paired data (17,149
contigs).

Application of Simplifier to the data for assembly of the Cp258,
from SOLiD (library of 50 bp fragments) data, eliminated up to
three bases at the ends from 8,004 contigs generated by Velvet
and Edena. After 10 minutes, 49 seconds processing with
Simplifier, the number of contigs was reduced to 5,272, which is
a reduction of 34.14%; in addition, N50 increased from 1 kb to
1.5 kb.

Therefore, Simplifier enabled us to remove redundant
Prokaryote sequences that were produced by genome assembly,
before the manual curation step, which reduced the time and
effort necessary for genome finalization. In addition, it allowed
us apply different *ab initio* methods for genome assembly and
then analyse only the unique sequences.

Acknowledgement:

This work was part of the Rede Paraense de Genômica e
Proteômica supported by Fundação de Amparo a Pesquisa do
Estado do Pará and Pronex Núcleo Amazônico de Excelência
em Genômica de Microorganismos. M.P.S, A.S., V.A. and
A.R.C. were supported by Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq). R.T.J.R.
acknowledges support from the Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior (CAPES).

References:

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[3] Alkan C *et al*. *Nat Methods*. 2010 **8**: 61 [PMID: 21102452]

[4] Metzker ML, *Nat Rev Genet*. 2010 **11**: 31 [PMID: 19997069]

[5] Silva A *et al*. *J Bacteriol*. 2011 **193**: 323 [PMID: 21037006]

[6] Hernandez D *et al*. *Genome Res*. 2008 **18**: 802 [PMID: 18332092]

[7] JNijkamp J *et al*. *Bioinformatics*. 2010 **26**: i433 [PMID: 20823304]

[8] Ramos RT *et al*. *BMC Res Notes*. 2011 **4**: 130 [PMID: 21501521]

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[10] Zerbino DR & Birney, Genome Res. 2008 18: 821 [PMID: 18349386]

Edited by P Kangueane

Citation: Ramos *et al*. Bioinformation 8(20): 996-999 (2012)

License statement: This is an open-access article, which permits unrestricted use, distribution, and reproduction in any medium, for non-commercial purposes, provided the original author and source are credited
**Supplementary material:**

<table><caption>**Table 1:** Results of the processing of fragment and mate-pair libraries</caption><thead><tr><th rowspan="2">Feature</th><th colspan="2">Fragments</th><th colspan="2">Mate-paired</th></tr><tr><th>Raw data</th><th>Simplifier</th><th>Raw data</th><th>Simplifier</th></tr></thead><tbody><tr><td>Number of Contigs</td><td>24,344</td><td>18,523</td><td>17,149</td><td>14,153</td></tr><tr><td>N50</td><td>552</td><td>642</td><td>697</td><td>717</td></tr><tr><td>Blastx vs NR(time)</td><td>109h36m</td><td>91h59m</td><td>103h4m</td><td>87h32m</td></tr><tr><td>Blast hits</td><td>4,171,658</td><td>3,178,597</td><td>3,309,206</td><td>2,734,811</td></tr></tbody></table>

NR - NCBI non-redundant databank. Comparative analysis of the number of contigs, value of N50, time for execution of Blast and quantity of hits generated, among the contigs originated from the mate-pair and fragment libraries of Escherichia coli DH10B before and after the application of Simplifier.

<table><caption>**Table 2:** Number of Escherichia coli DH10B contigs eliminated using Simplifier</caption><thead><tr><th rowspan="2">Library</th><th colspan="5">Size range (bp)</th></tr><tr><th>Up to 100</th><th>101-499</th><th>501-999</th><th>1,000-1,999</th><th>2,000-2,999</th></tr></thead><tbody><tr><td>Mate-paired</td><td>12</td><td>2,056</td><td>723</td><td>198</td><td>7</td></tr><tr><td>Fragments</td><td>117</td><td>5,134</td><td>517</td><td>53</td><td>0</td></tr></tbody></table>

Number of contigs removed by applying Simplifier to mate-paired and fragment library data, classified by contig length.