PLAST command-line arguments

Mandatory arguments

Each PLAST job requires at least to use the following arguments:

Argument Description
-p comparison method. One of: plastn, plastp, plastx, tplastn or tplastx
-i the query file provided as a Fasta formatted sequence file
-d the reference databank. Either a Fasta file or a BLAST databank
-o the results file

As mentioned in the table, PLAST is capable of directly working with Fasta files. However, in the context of the reference databank, you can also provide a databank name when you have made use of KDMS (Korilog Databank Manager System) that is provided with KlastRunner. Such a databank name can be obtained by having a look at the KDMS graphical frontend: on the right panel, have a look at the column called “Name”.

Controlling results size

You can control how many hits are reported in a result file by using these arguments:

Argument Description
-e E-value threshold. Default value is 10
-max-hit-per-query set the maximum number of hits aligned to a query. Default value is 10.
Requires to use additional argument: -force-query-order 1000
-max-hsp-per-hit set the maximum number of HSPs reported for a hit. Default value is 1.
Requires to use additional argument: -force-query-order 1000

If you want to get all possible hits/HSPs, simply pass 0 (zero) to -max-hit-per-query and -max-hsp-per-hit.

Sorting results by query IDs

Being a bank to bank sequence comparison tool, PLAST does not care about query ordering when producing results. It means that PLAST produces query/hit matches without any particular order. If you prefer PLAST producing results as BLAST, i.e. hits are sorted by query IDs, simply add the following argument to your PLAST command:

plast ... -force-query-order 1000 ...

Controlling speed

PLAST speed is of course controlled by the number of available cores:

Argument Description
-a Number of cores. Default is the maximum number of cores available on the computer running PLAST.

PLAST provides additional parameters to fine tune speed/quality ratio, as explained in the coming sections.

Optimizing quality/speed ratio using PLAST specific arguments

PLAST’s default configuration has been setup to provide an optimal ratio between speed and quality in order to produce results with quality similar to Blast. Even in such a configuration, you’ll have great speedup factors.

Depending on your needs you can enhance speed factors with little loss of quality in your results.

PLAST specific arguments for optimizing search jobs are:

Argument Description
-seeds-use-ratio Ratio of seeds to be used (see comment, below). [1..100], default is 100. Decrease value to speedup algorithm with little loss of quality
-s Ungapped threshold trigger a small gapped extension (see comment, below). [25..127], default is 38 and 55 for protein-based and nucleic-based comparisons, respectively. Increase value to speedup algorithm with little loss of quality.
-max-database-size Maximum allowed size (in bytes) for a database. If greater, database is segmented (see comment, below)

Fine tuning seed-ratio, threshold score and max-database-size may provide impressive acceleration of the KLAST comparison engine, with little loss of quality in the results. Carefuly read the following sections.

Optimizing PLAST: sample recipes

In order to tune PLAST correctly, we always invite our users to try the software with sample data sets. When you need to compare large set of sequences, always start your work by comparing a small subset of your data. This way, you can check the parameters, the results and the speed of the software.

As an example, if you have to compare 300,000 sequences against NCBI nt, start your work by comparing 300 query sequences against NCBI nt using default PLASTn parameters. Then, fine tune it (see below the use of seed-ratio, max-database-size and threshold score) and check the results. As soon as your parameters are fine, go ahead with 3,000 and/or 10,000 query sequences, and check results and speed. If everything is fine, then run the full comparison.

Optimizing PLAST at runtime: using seed-ratio

When using PLAST for protein-based sequence comparisons, the algorithm can be speedup using the seed-ratio parameter. As stated here, PLASTp algorithm relies on a finite table of seeds; there are about 6,200 seeds for BLOSUM50 and BLOSUM62 matrices, whatever the input sequence databanks (for more information, see Reference [1] here). During the comparison, PLAST orders seeds by occurrences, starting to process seeds producing the highest number of hits. So, it is possible to ask PLAST to use either the entire set of seeds to achieve a comparison, or a subset. This fine-tuning PLAST feature is achieved using the seed-ratio parameter, ranging from 1% to 100%. The highest seed-ratio you use, the highest sensitivity you get… the lowest seed-ratio you use, the highest speed you get with little loss in quality, as illustrated on this example:

Using seed ratio to speedup Plast

Using seed ratio to speedup Plast

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