Read Structures

NEW: Validate your read structures using this online tool.

• Overview
• Operators
• General Rules
• Examples
• Formal Grammar

Overview

In fgbio, fqtk, sgdemux, and also in Picard, a Read Structure refers to a String that describes how the bases in a sequencing run should be allocated into logical reads. It serves a similar purpose to the --use-bases-mask in Illumina's bcltofastq software, but provides some additional capabilities.

A Read Structure is a sequence of <number><operator> pairs or segments where, optionally, the last segment in the string is allowed to use + instead of a number for its length. The + means translates to whatever bases are left after the other segments are processed and can be thought of as meaning [0..infinity].

Read Structures are most commonly used in tools that convert from sequencer output formats (e.g. fastq files, BCLs) to downstream formats like SAM/BAM/CRAM, and in tools that process SAM/BAM/CRAM to extract non-template bases from the reads. Examples include:

• DemuxFastqs in fgbio to demultiplex a set of multi-sample fastq files and optionally extract UMIs
• FastqToBam in fgbio to convert from fastq to BAM while preserving sample barcode and UMI information
• ExtractUmisFromBam in fgbio which re-writes a BAM file with UMI sequences extracted from the reads and placed into tags
• IlluminaBasecallsToSam and IlluminaBasecallsToFastqin Picard both of which process BCLs and related files in an Illumina run folder and create BAMs or FASTQs respectively

Operators

Four kinds of operator are supported:

• T or Template: the bases in the segment are reads of template (e.g. genomic dna, rna, etc.)
• B or Sample Barcode: the bases in the segment are an index sequence used to identify the sample being sequenced
• M or Molecular Barcode: the bases in the segment are an index sequence used to identify the unique source molecule being sequence (i.e. a UMI)
• S or Skip: the bases in the segment should be skipped or ignored, for example if they are monotemplate sequence generated by the library preparation

General Rules

• Any number of segments >= 1 is valid
• The length of each segment must be a positive integer >= 1 (or +)
• Only the last segment in a read structure may use + for it's length
• Adjacent segments may use the same operator. E.g. if two sample indices are ligated onto a molecule separately such that they are adjacent, a structure of 6B6B+T is perfectly acceptable.

Examples

The following handful of examples attempt to describe the recommended way to describe a sequencing run in two different ways. Firstly as a single Read Structure for the entire run as you might use with IlluminaBasecallsToSam, and secondly as a set of Read Structures that would map one-to-one with the physical reads after fastq-conversion and optionally adapter trimming (which will create variable length reads):

• A simple 2x150bp paired end run with no sample or molecular indices:
  • 150T150T
  • [+T, +T]
• A 2x75bp paired end run with an 8bp I1 index read:
  • 75T8B75T
  • [+T, 8B, +T]
• A 2x150bp paired end run with an 8bp I1 index read and an inline 6bp UMI in read 1:
  • 6M142T8B150T
  • [6M+T, 8B, +T]
• A 2x150bp duplex sequencing run with dual sample-barcoding (I1 and I2) and both a 10bp UMI and 5bp monotemplate at the start of both R1 and R2:
  • 10M5S135T8B8B10M5S135T
  • [10M5S+T, 8B, 8B, 10M5S+T]

Formal Grammar

The formal grammar for Read Structures supported by fgbio is as follows:

<read-structure>     ::= <fixed-structure> <variable-segment>
<fixed-structure>    ::= "" | <fixed-length> <operator> <fixed-structure>
<variable-segment>   ::= "" | <variable-length> <operator>
<segment>            ::= <any-length><operator>
<operator>           ::= "T" | "B" | "M" | "S"
<fixed-length>       ::= <non-zero-digit>{<digit>}
<variable-length>    ::= "+"
<any-length>         ::= <fixed-length> / <variable-length>
<non-zero-digit>     ::= "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
<digit>              ::= "0" | <non-zero-digit>