g2p_mapper

A zero-dependency TypeScript library that maps genome positions to protein/transcript positions and back.

• Takes GFF3-style features with CDS subfeatures
• Handles forward and reverse strands
• Handles CDS phase offsets
• Deduplicates repeated CDS rows
• Handles codons that span an exon boundary

Install

npm install g2p_mapper

Usage

The input feature

genomeToTranscriptSeqMapping takes one transcript (an mRNA, say) as a plain object, with its CDS segments in subfeatures. Take this two-exon transcript in GFF3:

chr1  .  mRNA  100  205  .  +  .  ID=tx1
chr1  .  CDS   100  103  .  +  0  Parent=tx1
chr1  .  CDS   201  205  .  +  2  Parent=tx1

The equivalent feature object:

const feature = {
  refName: 'chr1',
  start: 99,
  end: 205,
  strand: 1,
  type: 'mRNA',
  subfeatures: [
    { refName: 'chr1', start: 99, end: 103, type: 'CDS', phase: 0 },
    { refName: 'chr1', start: 200, end: 205, type: 'CDS', phase: 2 },
  ],
}

• refName, start and end are required on every feature
• strand is required on the transcript, and is 1 or -1, not + or -
• Only subfeatures with type: 'CDS' count; exons, UTRs and the rest are ignored, and the CDS can be in any order
• phase is the GFF3 phase column; the mapper reads it from the first CDS in transcription order only
• Coordinates are 0-based and half-open, so a GFF3 start (1-based, inclusive) becomes start - 1 and end stays the same

g2p_mapper does not parse GFF3. Build the object from your parser's output; a JBrowse 2 feature's feature.toJSON() already has this shape, and so do the features from @gmod/gff-nostream.

With @gmod/gff-nostream

@gmod/gff-nostream returns features with 0-based coordinates, numeric strand and nested subfeatures, so its transcripts go to the mapper unchanged. The mapper wants the transcript, not the gene above it:

import { parseStringSync } from '@gmod/gff-nostream'
import { genomeToTranscriptSeqMapping } from 'g2p_mapper'

const gff = `chr1\t.\tgene\t100\t205\t.\t+\t.\tID=gene1
chr1\t.\tmRNA\t100\t205\t.\t+\t.\tID=tx1;Parent=gene1
chr1\t.\tCDS\t100\t103\t.\t+\t0\tParent=tx1
chr1\t.\tCDS\t201\t205\t.\t+\t2\tParent=tx1
`

for (const gene of parseStringSync(gff)) {
  for (const transcript of gene.subfeatures) {
    const { g2p, p2gCodon } = genomeToTranscriptSeqMapping(transcript)
    g2p[200] // 1
  }
}

Mapping positions

import { genomeToTranscriptSeqMapping, getCodonRanges } from 'g2p_mapper'

const { g2p, p2g, p2gCodon, refName, strand } =
  genomeToTranscriptSeqMapping(feature)

g2p[200] // 1: genome position 200 falls in the second amino acid
g2p[150] // undefined: 150 is intronic

p2g[1] // 102
p2gCodon[1] // [102, 200, 201]: this codon spans the intron

getCodonRanges(p2gCodon, 1) // [[102, 103], [200, 202]]
getCodonRanges(p2gCodon, 2) // [[202, 205]]

• g2p — genome position → protein position
• p2g — protein position → first genome position of the codon
• p2gCodon — protein position → every genome position of the codon, in transcription order
• getCodonRanges — a codon's genomic [start, end) ranges, one per contiguous piece, or undefined for a protein position outside the CDS

Protein positions are 0-based too: 0 is the first amino acid.

Reverse strand

With strand: -1 the mapper walks the CDS from the highest coordinate down, so the first amino acid sits at the right-hand end. The same transcript on the reverse strand has different phases, because the right-hand CDS now comes first:

const { g2p, p2g, p2gCodon } = genomeToTranscriptSeqMapping({
  ...feature,
  strand: -1,
  subfeatures: [
    { refName: 'chr1', start: 99, end: 103, type: 'CDS', phase: 1 },
    { refName: 'chr1', start: 200, end: 205, type: 'CDS', phase: 0 },
  ],
})

g2p[204] // 0: the highest CDS base starts the protein
g2p[99] // 2

p2g[1] // 201: the codon's first base, which is its highest coordinate
p2gCodon[1] // [201, 200, 102]: transcription order, so descending

getCodonRanges(p2gCodon, 1) // [[102, 103], [200, 202]]: always ascending

Docs

• mapping.md — how the maps are built, with a flowchart, and how reverse strands and split codons behave
• api.md — the Feat input, the return maps, and a worked example

See also

• g2p_mapper_cli — CLI wrapper for this library
• interproscan2genome — maps InterProScan protein annotations back to genome coordinates
• jbrowse-plugin-protein3d — maps 3-D protein structure positions to the genome
• jbrowse-plugin-msaview — maps MSA sequence positions to the genome

Footnote

g2p_mapper assumes simple 3-letter codon translation, which does not always hold. Validate this assumption against your own data.

Publishing

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