A New Approach for Bacterial Genome Annotation Designed for Next Generation Sequencing Data
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BG7: A New Approach for Bacterial Genome Annotation Designed for Next Generation Sequencing Data Pablo Pareja-Tobes, Marina Manrique, Eduardo Pareja-Tobes, Eduardo Pareja, Raquel Tobes* Oh no sequences! Research group, Era7 Bioinformatics, Granada, Spain Abstract BG7 is a new system for de novo bacterial, archaeal and viral genome annotation based on a new approach specifically designed for annotating genomes sequenced with next generation sequencing technologies. The system is versatile and able to annotate genes even in the step of preliminary assembly of the genome. It is especially efficient detecting unexpected genes horizontally acquired from bacterial or archaeal distant genomes, phages, plasmids, and mobile elements. From the initial phases of the gene annotation process, BG7 exploits the massive availability of annotated protein sequences in databases. BG7 predicts ORFs and infers their function based on protein similarity with a wide set of reference proteins, integrating ORF prediction and functional annotation phases in just one step. BG7 is especially tolerant to sequencing errors in start and stop codons, to frameshifts, and to assembly or scaffolding errors. The system is also tolerant to the high level of gene fragmentation which is frequently found in not fully assembled genomes. BG7 current version – which is developed in Java, takes advantage of Amazon Web Services (AWS) cloud computing features, but it can also be run locally in any operating system. BG7 is a fast, automated and scalable system that can cope with the challenge of analyzing the huge amount of genomes that are being sequenced with NGS technologies. Its capabilities and efficiency were demonstrated in the 2011 EHEC Germany outbreak in which BG7 was used to get the first annotations right the next day after the first entero-hemorrhagic E. coli genome sequences were made publicly available. The suitability of BG7 for genome annotation has been proved for Illumina, 454, Ion Torrent, and PacBio sequencing technologies. Besides, thanks to its plasticity, our system could be very easily adapted to work with new technologies in the future. Citation: Pareja-Tobes P, Manrique M, Pareja-Tobes E, Pareja E, Tobes R (2012) BG7: A New Approach for Bacterial Genome Annotation Designed for Next Generation Sequencing Data. PLoS ONE 7(11): e49239. doi:10.1371/journal.pone.0049239 Editor: Ramy K. Aziz, Cairo University, Egypt Received March 27, 2012; Accepted October 9, 2012; Published November 21, 2012 Copyright: ß 2012 Pareja-Tobes et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work has been partially funded by the PID 1465 project of the Agencia de Innovacio´n y Desarrollo de Andalucı´a and by Grant ‘‘brachVac’’ IPT-2011- 0735-010000 from Spanish Ministerio de Ciencia e Innovacio´n. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: Era7 offers Service of Bacterial annotation based on BG7, but BG7 code is available at GitHub: https://github.com/bg7/ under the license AGPLv3. All authors work at the research group named Oh no sequences! within Era7 Bioinformatics company. This does not alter the authors’ adherence toall the PLOS ONE policies on sharing data and materials. * E-mail: [email protected] Introduction have improved the read length and the sequencing speed but the final sequences continue bearing errors. Thus Ion Torrent The massive production of bacterial genome sequences using technology has errors similar to those described for 454 [3], and NGS technologies is demanding new automated systems capable PacBio technology [4] provides sequences with indels and of getting an accurate annotation of a complete genome in a short substitutions, although, due to its random error profile, it has enough time. Classical annotation systems are based on two many possibilities for improving current error rates. The new read completely separate phases: 1) ORF prediction and 2) Functional sizes of around thousands of bases that provide third generation annotation. Classical ORF prediction methods are totally depen- technologies as Pacbio open new strategies for assembly. dent on the detection of start and stop codons. This is an efficient This scenario demands new approaches better fitted to the new strategy whenever the sequencing technology has minimal sequencing technologies framework. sequence errors since only predicted ORFs have to be annotated After analyzing the publications about bacterial annotation - avoiding an important amount of unnecessary comparisons. pipelines we found that the use of the classical genome annotation However, all NGS technologies generate sequences with substitu- paradigm - composed by a first phase of ORF prediction and a tion, deletion and insertion errors. Each technology is prone to subsequent phase of functional annotation, continues being the generate different types of errors: 454 technology generates deletions and insertions at homopolymeric regions [1] whilst Solid standard also for NGS annotation. and Illumina technologies [2] generate substitutions, especially DOE-JGI MAP [5] uses GeneMark program [6] to predict when coverage is not sufficient to correct the exact base at each ORFs and DIYA [7], AGeS [8], RAST [9], JCVI [10], ISGA position in the final consensus. Irregularity in coverage is [11], BAsys [12] and WeGAS [13] use Glimmer [14] to predict sometimes the reason for the presence of regions with abundant the putative genes in the first phase. All of them follow the classical errors in the final consensus. In addition, in NGS genome projects paradigm consisting of a first independent phase of ORF the assembly is frequently uncompleted and a significant amount prediction and a second independent phase of annotation. It is of genes can remain fragmented. Third generation technologies certainly possible to get good quality annotation for NGS genome PLOS ONE | www.plosone.org 1 November 2012 | Volume 7 | Issue 11 | e49239 BG7:A New Approach for Bacterial Genome Annotation projects using this kind of systems but they are based on a value amongst those from the merged HSPs is the E value for that paradigm that was designed for Sanger sequences. gene. RAST [9] is one of the most used servers for bacterial genome Once the compatible HSPs are joined, the region of similarity is annotation. It identifies protein-encoding and RNA genes and extended upstream and downstream searching for start and stop assigns functions and subsystems to reconstruct the metabolic codons taking into account the orientation of the gene based on networks in which the genes are involved. It supports comparative the alignment with the reference protein. analysis with the annotated genomes maintained in the SEED In the case where stop or start codons are not found within a environment. A comparison with RAST was carried out analyzing fixed distance, the gene is considered as a gene with ‘‘non- false positives and false negatives in the BG7 and RAST canonical’’ start or/and stop. This strategy prevents gene loss annotation of two very different assemblies of the Germany when there is a lack of either stop or start codons caused by outbreak E. coli. The first assembly had 3057 contigs and a high sequencing errors. If a sufficient similarity supports the existence of error rate and the last assembly had 4 contigs and a very low error a gene, BG7 maintains this region as a gene even though the start rate. The annotation used for determining the correct genes was and end of the genes are not well defined. This strategy also solves the BROAD annotation of the last assembly that included 5164 the problem of detection of not complete genes located at the ends genes (The BROAD reference genes used in the test are in File of contigs. S1). For the first assembly (named BV1) BG7 yielded 6190 genes In the case where the same contig region is similar to several with 197 false negatives and 186 false positives and RAST 8253 different reference proteins, the best hit (hit with the lowest e- genes with 247 false negatives and 455 false positives. For the last value) is chosen. Once all similarity-supported putative genes are assembly (named BV4) the data for BG7 was 5210 genes, 163 false defined, the system solves the overlapping between them, selecting negatives and 271 false positives while the number of genes the better gene for each sequence fragment. So each final gene is obtained with RAST was 5446 with 116 false negatives and 321 supported by the similarity with a specific and unique Uniprot false positives. (See Tables S1, S2). BAsys [12] provides 60 protein. A threshold for gene overlapping can be set so gene annotation fields for each gene and a fully navigable graphical overlaps beyond the fixed threshold are solved based on the E map, which is hyperlinked to textual gene descriptions but it is not value of each putative gene and on the provenance of the appropriate to annotate very fragmented genomes. annotating protein. When the overlapping genes have different E Using the classical paradigm - ORF prediction first and then values, the gene with the lowest E value is chosen.Otherwise, when gene annotation, any gene lost in the first phase cannot be they have the same E-value, the one from the closest organism recovered in later phases of the annotation. isselected. The closest organism is inferred by analyzing the Specifically, when using Glimmer, any minimal error affecting tBLASTn results in a preprocessing step. The system evaluates start or stop codons can cause unrecoverable gene loss or false which organism has more ortholog proteins in the genome under gene prediction. BG7 has been designed for NGS based on a new analysis, assuming then the closest organism as the preferential one for the annotation process.