matchPWM {Biostrings} | R Documentation

## PWM creating, matching, and related utilities

### Description

Position Weight Matrix (PWM) creating, matching, and related utilities for DNA data. (PWM for amino acid sequences are not supported.)

### Usage

    PWM(x, type = c("log2probratio", "prob"),
        prior.params = c(A=0.25, C=0.25, G=0.25, T=0.25))

    matchPWM(pwm, subject, min.score="80%", with.score=FALSE, ...)
    countPWM(pwm, subject, min.score="80%", ...)
    PWMscoreStartingAt(pwm, subject, starting.at=1)

    ## Utility functions for basic manipulation of the Position Weight Matrix
    maxWeights(x)
    minWeights(x)
    maxScore(x)
    minScore(x)
    unitScale(x)
    ## S4 method for signature 'matrix'
    reverseComplement(x, ...)

### Arguments

`x` | For `PWM`: a rectangular character vector or rectangular DNAStringSet object ("rectangular" means that all elements have the same number of characters) with no IUPAC ambiguity letters, or a Position Frequency Matrix represented as an integer matrix with row names containing at least A, C, G and T (typically the result of a call to `consensusMatrix`). For `maxWeights`, `minWeights`, `maxScore`, `minScore`, `unitScale` and `reverseComplement`: a Position Weight Matrix represented as a numeric matrix with row names A, C, G and T.
`type` | The type of Position Weight Matrix, either "log2probratio" or "prob". See Details section for more information.
`prior.params` | A positive numeric vector, which represents the parameters of the Dirichlet conjugate prior, with names A, C, G, and T. See Details section for more information.
`pwm` | A Position Weight Matrix represented as a numeric matrix with row names A, C, G and T.
`subject` | Typically a DNAString object. A Views object on a DNAString subject, a MaskedDNAString object, or a single character string, are also supported. IUPAC ambiguity letters in `subject` are ignored (i.e. assigned weight 0) with a warning.
`min.score` | The minimum score for counting a match. Can be given as a character string containing a percentage (e.g. `"85%"`) of the highest possible score or as a single number.
`with.score` | `TRUE` or `FALSE`. If `TRUE`, then the score of each hit is included in the returned object in a metadata column named `score`. Say the returned object is `hits`, this metadata column can then be accessed with `mcols(hits)$score`.
`starting.at` | An integer vector specifying the starting positions of the Position Weight Matrix relatively to the subject.
`...` | Additional arguments for methods.

### Details

The `PWM` function uses a multinomial model with a Dirichlet conjugate prior to calculate the estimated probability of base b at position i. As mentioned in the Arguments section, `prior.params` supplies the parameters for the DNA bases A, C, G, and T in the Dirichlet prior. These values result in a position independent initial estimate of the probabilities for the bases to be `priorProbs = prior.params/sum(prior.params)` and the posterior (data infused) estimate for the probabilities for the bases in each of the positions to be `postProbs = (consensusMatrix(x) + prior.params)/(length(x) + sum(prior.params))`. When `type = "log2probratio"`, the PWM = `unitScale(log2(postProbs/priorProbs))`. When `type = "prob"`, the PWM = `unitScale(postProbs)`.

### Value

A numeric matrix representing the Position Weight Matrix for `PWM`.

A numeric vector containing the Position Weight Matrix-based scores for `PWMscoreStartingAt`.

An XStringViews object for `matchPWM`.

A single integer for `countPWM`.

A vector containing the max weight for each position in `pwm` for `maxWeights`.

A vector containing the min weight for each position in `pwm` for `minWeights`.

The highest possible score for a given Position Weight Matrix for `maxScore`.

The lowest possible score for a given Position Weight Matrix for `minScore`.

The modified numeric matrix given by `(x - minScore(x)/ncol(x))/(maxScore(x) - minScore(x))` for `unitScale`.

A PWM obtained by reverting the column order in PWM `x` and by reassigning each row to its complementary nucleotide for `reverseComplement`.

### Author(s)

H. Pagès and P. Aboyoun

### References

Wasserman, WW, Sandelin, A., (2004) Applied bioinformatics for the identification of regulatory elements, Nat Rev Genet., 5(4):276-87.

### See Also

`consensusMatrix`, `matchPattern`, `reverseComplement`, DNAString-class, XStringViews-class

### Examples

    ## Data setup:
    data(HNF4alpha)
    library(BSgenome.Dmelanogaster.UCSC.dm3)
    chr3R <- Dmelanogaster$chr3R
    chr3R

    ## Create a PWM from a PFM or directly from a rectangular
    ## DNAStringSet object:
    pfm <- consensusMatrix(HNF4alpha)
    pwm <- PWM(pfm)  # same as 'PWM(HNF4alpha)'

    ## Perform some general routines on the PWM:
    round(pwm, 2)
    maxWeights(pwm)
    maxScore(pwm)
    reverseComplement(pwm)

    ## Score the first 5 positions:
    PWMscoreStartingAt(pwm, chr3R, starting.at=1:5)

    ## Match the plus strand:
    hits <- matchPWM(pwm, chr3R)
    nhit <- countPWM(pwm, chr3R)  # same as 'length(hits)'

    ## Use 'with.score=TRUE' to get the scores of the hits:
    hits <- matchPWM(pwm, chr3R, with.score=TRUE)
    head(mcols(hits)$score)
    min(mcols(hits)$score / maxScore(pwm))  # should be >= 0.8

    ## The scores can also easily be post-calculated:
    scores <- PWMscoreStartingAt(pwm, subject(hits), start(hits))

    ## Match the minus strand:
    matchPWM(reverseComplement(pwm), chr3R)

[Package Biostrings version 2.60.2 Index]