primer design based on parameterized sweeps and constraint filtering
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SWGA is a tool for choosing primers for the selective amplification of a target genome from a sample containing a mixture of target and contaminating DNA (i.e. pathogen genome from infected host blood) [cite relevant paper]. It does so by identifying short, recurring motifs in a target sequence file and scoring sets of motifs based on selectivity for and even distribution in the target sequence against a background sequence file.


Table of Contents


To use this you'll need:

  • A Unix environment
    • GNU/Linux works out of the box. Debian + SUSE tested.
    • Cygwin has been tested. gcc and python are required, might need to run rebasall
    • OS/X support. not yet!
  • dna-utils
  • bash or compliant shell.
  • python 2.7.x


git clone
cd SelectiveWholeGenomeAmplification
sudo make install

Example Usage

Standard use of (SGA) SelectiveWholeGenomeAmplification is easy. it takes two arguments, the foreground and background

SelectiveWholeGenomeAmplification PfalciparumGenome.fasta HumanGenome.fasta;
less PfalciparumGenome_HumanGenome/final_mers

SWGA User Interface

SWGA also comes with a easy to use user prompt called SelectiveWholeGenomeAmplificationUI. It allows for a less experienced user to use SWGA without issue. to run this all you need to do is run SelectiveGenomeAmiplifcationUI and you'll see a series of prompts asking the user about tunables like below

Where would you like to temporary files to be stored? (Default=$output_directory/.tmp): 
Where would you like to count files to be stored? (Default=$output_directory/.tmp): 
maximum mer size you would like to pick? (Default=12): 10
minimum mer size you would like to pick? (Default=6): 7
eliminate mers that appear less frequently on average than this number ? (Default=50000): 25000
Input the path to your foreground file:target.fa  
Input the path to your background file:humangenome.fa 
Would you like to output your inserted variables to a string you can later paste? (Y/N/Default=y): n
Run SelectiveWholeGenomeAmplification? (Y/N/Default=y): y

Setting Tunable Parameters

SGA allows for many tunable parameters, which are all explained in the chart below. For user customizable variables, they need to be passed in as environmental variables like so:

max_mer_distance=5000 max_select=6 min_mer_range=6 max_mer_range=12 \ PfalciparumGenome.fasta half.fasta 

Running individual steps

By default SelectiveWholeGenomeAmplification runs all four steps, but you can specify the program to run other steps, like in these examples.

current_run=run_1 SelectiveWholeGenomeAmplification target.fasta bg.fasta score

current_run=run_1 SelectiveWholeGenomeAmplification target.fasta bg.fasta select score

current_run=run_1 SelectiveWholeGenomeAmplification target.fasta bg.fasta 3 4 

valid steps are these:

  • count (1)
  • filter (2)
  • select (3)
  • score (4)

This function does not try to be smart, so use it wisely.

Manually scoring specific mer combinations from list

Users can manually score combinations of mers they choose using the script. -f foreground.fa -b background.fa -c combination file -o output

The combination file should look like this:


each row is combination to be scored.

Manually score all combinations from list

Users can manually score all combinations of mers they choose using the script. -f foreground.fa -b background.fa -m mer file -o output

The mer file should look like this:


each row is a separate mer. do not put multiple mers on one line.

Manually rescore all combinations from previously scored file

Users can manually rescore all combinations of mers they previously used in the script. This allows users to test different score functions easily with the same combinations.

An example would be this:

score_func=nb_primers**2 -f fg.fa -b bg.fa -r fg_bg/run_1/all-scores -o primers_squared_scores

Tunable Parameters

variable | default | notes :---- | :---- | ---- | :---- current_run | Not Enabled | specify the run you want to run steps on min_mer_range | 6 | minimum mer size to use max_mer_range | 12 | maximum mer size to use max_mer_distance | 5000 | maximum distance between mers in foreground min_melting_temp | 0° | minimum melting temp of mers max_melting_temp | 30° | maximum melting temp of mers min_foreground_binding_average | 50000 | eliminate mers that appear less frequently than the average (length of foreground / # of occurrances) min_bg_ratio | Not Enabled | eliminate mers where the background ration is less than the minimum ignore_mers | Not Enabled | mers to explicitly ignore, space separated ex. ignore_mers="ACAGTA ACCATAA ATATATAT" ignore_all_mers_from_files | Not Enabled | ignore any mers found in these files. space separated. output_directory | $foreground_$background/ | ex. if fg is Bacillus.fasta and bg is HumanGenome.fasta then folder would be $PWD/Bacillus.fasta_HumanGenome_output.fasta/ counts_directory | $output_directory/.tmp | directory for counts directory tmp_directory | $output_directory/.tmp | temporary files directory max_select | 15 | maximum number of mers to pick max_check | 35 | maximum number of mers to select (check the top #) foreground | Not Enabled | path of foreground file background | Not Enabled | path of background file max_consecutive_binding | 4 | The maximum number of consecutive binding nucleotides in homodimer and heterodimers fg_weight | 0 | How much extra weight to give higher frequency mers in fg. see "equations" (between 0 and 1) primer_weight | 0 | How much extra weight to give to sets with a higher number of primers. (between 0 and 1) output_top_nb | 10000 | How many scores do you want to output in your sorted output file? score_func | Not Enabled | see the custom scoring section sort_by | min | How do you want to rank top-scores? min means smaller is better, max is larger. 'min' or 'max'


Here's what we are using to determine our scoring and selectivity

Mer Selectivity

Our selectivity is what we use to determine what top $max_check mers are checked later on in our scoring function. Currently we use this formula:

By default our fg_weight is zero. This gives no extra weight to more frequently occurring mers, but can be set higher with the fg_weight environmental variable if you wish to do so.

hit = abundance of primer X (ex. 'ATGTA') in background

(foreground hit / background hit) * (foreground hit ^ fg_weight)

Scoring combinations

All variables used in our scoring function are described here:

fg_pts = an array of all the points of each mer in the combination, and sequence ends
fg_mean_dist = mean distance between each point in fg_pts
fg_stddev = standard deviation of distance between each point in fg_pts

nb_primers = number of primers in a combination
primer_weight = extra weight for sets with higher primers

bg_ratio = length of background / number of times primer was in background

Default scoring function

The default scoring function is this:

mer_score = (nb_primers**primer_weight) * (fg_mean_dist * fg_std_dist) / bg_ratio

Custom scoring function

We support custom scoring via python's exec methods. This means that you can destroy your system, blow up the universe, implode your hard drive, all within the confines of this exec. That means don't do anything crazy. Stick to basic arithmetic.

This is a security hole.

you can specify it like any other parameter like so:

# the default function
score_func="(nb_primers**primer_weight) * (fg_mean_dist * fg_std_dist) / bg_ratio"

You need to use valid python code.


There are several filters that our mers go through, to eliminate ones that won't fit our needs. They are all configurable via the tunable parameters. If you look in a output directory, you'll see a folder called "passes-filter". This contains a file for each of the different steps in the pipeline, and the contents of each file is what 'passes' that filter.

For example, if you ignored the mer 'AAAAA', then in passes-filter/1-$foreground-ignore-mers there would be no line containing that.

The filter system works like a big pipe, whatever gets filtered out won't make it to the next step. the order is like this

All mers -> ignore_mers -> ignore_all_mers -> average_binding -> non_melting -> consecutive_binding


The file structure outputted by default is this:

└── run_1 # current_run
    ├── passes-filter # filter folder for filtering steps
    │   ├── 1-$foreground-ignore-mers
    │   ├── 2-$foreground-ignore-all-mers
    │   ├── 3-$foreground-average-binding
    │   ├── 4-$foreground-non-melting
    │   └── 5-$foreground-consecutive-binding
    ├── $foreground-filtered-counts # final filtered mers used for
    ├── parameters # parameters used in the run
    ├── selected-mers # final filtered mers used for
    ├── selected-mers # final filtered mers used for
    ├── all-scores    # file outputted by (all the scores generated)
    └── top-scores    # the sorted top $output_top_nb scores from all-scores output

Select mers outputs a tab delimited file, with 4 columns: mer, foreground count, background count, and the mer selectivity value. (higher is better)

CTAACTTAGGTC  1572  155  10.14194
CTAACATAGGTC  1479  132  11.20455
GACCTATGTTAG  1479  132  11.20455 output

score mers outputs a tab delimited file with 6 columns:

nb_primers  Combination  Score  FG_mean_dist  FG_stdev_dist  BG_ratio

Post Processing

To get a more detailed look at each scored combination we provide the script. This script will output all of the points in a selected set along with some metadata, including position, what sequence it is in, what strand and what mer it is. -f fg.fa -s fg.fa_bg.fa/run_12/top-scores -n 15 -o sets

this will output one file for eat of the the top 15 sets in top-scores, in the folder sets.