Gene annotation I: BLAST-like and HMM

Objectives

• Annotation methods
• Annotating MAGs against the UniProt database with DIAMOND
• Annotating MAGs against the Pfam database with HMMER
• Evaluating the quality of gene assignment
• Differences in taxonomies

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Annotation methods

Broadly speaking, there are two ways we perform gene annotations with protein sequences. Both compare our sequences of interest against a curated set of protein sequences for which function is known, or is strongly suspected. In each case, there are particular strengths to the approach and for particular research questions, one option may be favoured over another.

BLAST-like annotation

The first of these is the BLAST algorithm for sequence alignment. This approach performs pairwise alignment between the gene of interest (query sequence) and the sequences in the database (target sequence). BLAST searches each potential target sequence for k-mers identified in the query sequence. Where these k-mers are found in targets, the ends are extended out to try to create longer regions of highly similar sequence spans. Across this span, the tool identifies the longest span of characters (nucleotide or amino acid) that match within a scoring framework to return the length of the region (coverage) and the sequence identity over the span (identity).

The original tool for performing this kind of analysis was the BLAST tool. While BLAST and its variants are still excellent tools for performing this kind of sequence annotation, they suffer from a slow runtime speed due to the need to test each query sequence against every target sequence in the database. For this reason, several tools have been published which take the basic approach of BLAST, but augment it with methods to reduce the number of pairwise comparisons needed to identify targets with high sequence similarity to the query. Two popular pieces of software are the tools USEARCH and DIAMOND.

HMM-profiling of domains

An alternate method for attributing function to query sequences is to consider them as a collection of independently functioning protein folding domains. This is the approach used in the HMMER software, and the Pfam, TIGRfam, and PANTHER databases. In these analyses, the database consists not of individual sequences, but of Hidden Markov models built from a collection of proteins that share a common domain. These profiles build out a statistical map of the amino acid transitions (from position to position), variations (differences at a position), and insertions/deletions between positions in the domain across the different observations in the training database and apply these maps to the query data.

These exercises will take place in the 10.gene_annotation_and_coverage/ folder.

Navigate to working directory

cd /nesi/nobackup/nesi02659/MGSS_U/<YOUR FOLDER>/10.gene_annotation_and_coverage

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Annotating MAGs against the UniProt database with DIAMOND

For this exercise we are going to use DIAMOND for performing our annotation. We have chosen to use this tool because it is faster than BLAST, and USEARCH comes with licensing restrictions that make it hard to work with in a shared computing environment like NeSI.

For this exercise we have created a diamond-compatible database from the 2018 release of the UniProt database.

For input files, the predictions/ results from the previous gene prediction exercise have been copied over to 10.gene_annotation_and_coverage/predictions/.

In general, diamond takes a pair of input files - the protein coding sequences we wish to annotate and the database we will use for this purpose. There are a few parameters that need to be tweaked for obtaining a useful output file, however.

code

module purge
module load DIAMOND/2.0.15-GCC-11.3.0

diamond help

Terminal output

diamond v2.0.15.153 (C) Max Planck Society for the Advancement of Science
Documentation, support and updates available at http://www.diamondsearch.org
Please cite: http://dx.doi.org/10.1038/s41592-021-01101-x Nature Methods (2021)

Syntax: diamond COMMAND [OPTIONS]

Commands:
...
blastp  Align amino acid query sequences against a protein reference database
...

General options:
--threads (-p)         number of CPU threads
--db (-d)              database file
--out (-o)             output file
--outfmt (-f)          output format
...

There are two output formats we can chose from which are useful for our analysis. We will obtain our output in the BLAST tabular format, which provides the annotation information in a simple-to-parse text file that can be viewed in any text or spreadsheet viewing tool. This will allow us to investigate and evaluate the quality of our annotations.

Awkwardly, DIAMOND does not provide the headers for what the columns in the output table mean. This table is a handy reference for how to interpret the output.

From here we can view important statistics for each query/target pairing such as the number of identical residues between sequences and the aligned length between query and target.

Before we begin, we need to create an directory for outputs.

code

mkdir -p gene_annotations

Now, lets set up a slurm job to annotate each of our MAGs.

Create a new script

Create script named annotate_uniprot.sl

nano annotate_uniprot.sl

Remember to update to your own folder

code

#!/bin/bash -e

#SBATCH --account       nesi02659
#SBATCH --job-name      annotate_uniprot
#SBATCH --partition     milan
#SBATCH --time          01:00:00
#SBATCH --mem           20GB
#SBATCH --cpus-per-task 20
#SBATCH --array         0-9
#SBATCH --error         %x_%A_%a.err
#SBATCH --output        %x_%A_%a.out

# Load modules
module purge
module load DIAMOND/2.0.15-GCC-11.3.0

# Working directory
cd /nesi/nobackup/nesi02659/MGSS_U/<YOUR FOLDER>/10.gene_annotation_and_coverage

# Variables
prot_file=predictions/bin_${SLURM_ARRAY_TASK_ID}.filtered.genes.no_metadata.faa
out_file=$(basename ${prot_file} .faa)
db=/nesi/nobackup/nesi02659/MGSS_resources_2022/databases/uniprot.20181026.dmnd

# Run DIAMOND
diamond blastp --threads $SLURM_CPUS_PER_TASK --max-target-seqs 5 --evalue 0.001 \
               --db $db --query ${prot_file} --outfmt 6 \
               --out gene_annotations/${out_file}.uniprot.txt

Submit the script

sbatch annotate_uniprot.sl

---

Annotating MAGs against the Pfam database with HMMER

The standard software for performing HMM-profiling annotation is HMMER. Compared to BLAST, FASTA, and other sequence alignment and database search tools based on older scoring methodology, HMMER aims to be significantly more accurate and more able to detect remote homologs because of the strength of its underlying mathematical models. In the past, this strength came at significant computational expense, but in the new HMMER3 project, HMMER is now essentially as fast as BLAST.

HMMER will search one or more profiles against a sequence database for sequence hommologs, and for making sequence alignments, implementing profile hidden Markov models. In this exercise, we will perform a search using hmmsearch. For each profile in hmmfile, HMMER uses that query profile to search the target database of sequences indicated in seqdb, and output ranked lists of the sequences with the most significant matches to the profile. hmmsearch accepts any fastA file as target database input. It also accepts EMBL/UniProtKB text format, and Genbank format. It will automatically determine what format your file is in so you don’t have to specify it.

As we did with diamond, we will also have to modify some parameters to get the desired ouotput.

code

module load HMMER/3.3.2-GCC-11.3.0

hmmsearch -h

hmmsearch -h

hmmsearch :: search profile(s) against a sequence database
HMMER 3.3.2 (Nov 2020); http://hmmer.org/
Copyright (C) 2020 Howard Hughes Medical Institute.
Freely distributed under the BSD open source license.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Usage: hmmsearch [options] <hmmfile> <seqdb>

Basic options:
 -h : show brief help on version and usage

Options directing output:
...
--tblout <f>     : save parseable table of per-sequence hits to file <f>
....

Options controlling reporting thresholds:
...
-E <x>     : report sequences <= this E-value threshold in output  [10.0]  (x>0)
...

Other expert options:
...
--cpu <n>     : number of parallel CPU workers to use for multithreads
...

We are now going to submit another slurm job to annotate our MAGs using the Pfam database. Pfam used to have a standalone website, but it has recently been integrated into InterPro maintained by the European Bioinformatics Institute (see announcement). Matching sequences to a Pfam entry allows us to transfer the functional information from an experimentally characterised sequence to uncharacterised sequences in the same entry. Pfam then provides comprehensive annotation for each entry.

Create script named annotate_pfam.sl

nano annotate_pfam.sl

Remember to update to your own folder

code

#!/bin/bash -e

#SBATCH --account       nesi02659
#SBATCH --job-name      annotate_pfam
#SBATCH --partition     milan
#SBATCH --time          00:20:00
#SBATCH --mem           5GB
#SBATCH --cpus-per-task 10
#SBATCH --array         0-9
#SBATCH --error         %x_%A_%a.err
#SBATCH --output        %x_%A_%a.out

# Load modules
module purge
module load HMMER/3.3.2-GCC-11.3.0

# Working directory
cd /nesi/nobackup/nesi02659/MGSS_U/<YOUR FOLDER>/10.gene_annotation_and_coverage

# Variables
prot_file=predictions/bin_${SLURM_ARRAY_TASK_ID}.filtered.genes.no_metadata.faa
out_file=$(basename ${prot_file} .faa)
db=/nesi/nobackup/nesi02659/MGSS_resources_2022/databases/Pfam-A.hmm

# Run HMMER
hmmsearch --tblout gene_annotations/${out_file}.pfam.txt -E 0.001 \
          --cpu $SLURM_CPUS_PER_TASK \
          ${db} ${prot_file}

Submit the script

sbatch annotate_pfam.sl

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Evaluating the quality of gene assignment

Determining how trustworthy a gene annotation is can be a very tricky process. How similar do protein sequences need to be to perform the same function? The answer is surprisingly low. A bioinformatic analysis performed in 1999 identified that proteins with as little as 20 - 35% sequence identity can still share the same function (Rost, 1999), but this is not a universal occurrence. When evaluating annotations, consider the following questions:

1. What is the amino acid identity along the aligned region?
2. What is the amino acid similarity between the aligned region?
3. What is the coverage as a percentage of the query and target genes?
4. If we infer a phylogeny of this query gene with references from the target family, is a stable tree resolved?
5. Does the inclusion of this gene function make sense in the context of the organism's taxonomy?
6. Does the gene sit on a long contig that is core to the MAG, or is it a short contig carrying only a single gene?
7. If we are uncertain of a particular annotation, does the predicted gene occur in an operon? If so, are the other genes present in the annotation?

We must also remain aware of the potential for incorrectly annotated genes in the annotation database and that proteins can perform multiple functions (and may therefore be attributed multiple, inconsistent annotations). Furthermore, it is also important to consider exactly which part of the target gene the alignment is happening across. There are several catalytic centers of enzymes, such as the Fe-S cofactor, which are shared across many different proteins, and if your annotation is only spanning one of these regions then it may simply be the case that you are identifying a generic electron accepting or donating domain.

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Differences in taxonomies

Another way to determine if an annotation 'belongs' in the MAG of interest is to consider the predicted taxonomy of the query gene with that of the MAG itself. For example, if you detect a Desulfovibrio-like dsrA sequence in a bin that has been classified as belonging to the genus Desulfovibrio then it is probably a safe bet that the annotation is correct.

However, when comparing taxonomic assignments, it is important to be aware of the differing taxonomic schemas that are circulating in the microbiological and bioinformatic literature and to know how to reconcile their differences. Similar to how the 16S rRNA gene taxonomies provided by SILVA, Greengenes, and RDP taxonomies all differ in some aspects, there are multiple competing taxonomies in protein databases.

This problem exists despite the existence of a formal Code for the naming of bacteria and archaea, because

1. There are no rules governing how we define the grouping of these names together, other than for type species
2. Defunct synonyms and basonyms are not correctly purged from taxonomy lists (this is quite noticeable with the NCBI taxonomy)
3. Valid names cannot be assigned for uncultivated organisms, meaning there are many informal placeholder names in the literature. For example, clades like WPS-2, SAR324, and SAUL are widely cited in the literature despite having no official standing

It is therefore important to periodically sanity check your taxonomic annotations in order to avoid splitting taxa based on spelling differences or the use of historic names that have since been reclassified.

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