Section: Mathematical & Computational Biology
Topic: Genetics/Genomics, Applied mathematics

General encoding of canonical k-mers

10.24072/pcjournal.323 - Peer Community Journal, Volume 3 (2023), article no. e87.

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To index or compare sequences efficiently, often k-mers, i.e., substrings of fixed length k, are used. For efficient indexing or storage, k-mers are often encoded as integers, e.g., applying some bijective mapping between all possible σk k-mers and the interval [0, σk −1], where σ is the alphabet size. In many applications, e.g., when the reading direction of a DNA-sequence is ambiguous, canonical k-mers are considered, i.e., the lexicographically smaller of a given k-mer and its reverse (or reverse complement) is chosen as a representative. In naive encodings, canonical k-mers are not evenly distributed within the interval [0, σk −1]. We present a minimal encoding of canonical k-mers on alphabets of arbitrary size, i.e., a mapping to the interval [0, σk/2−1]. The approach is introduced for canonicalization under reversal and extended to canonicalization under reverse complementation. We further present a space and time efficient bit-based implementation for the DNA alphabet.

Published online:
DOI: 10.24072/pcjournal.323
Type: Research article
Keywords: k-mers, canonical k-mers, encoding, minimal perfect hash function
Wittler, Roland 1

1 Faculty of Technology, Bielefeld Institute for Bioinformatics Infrastructure (BIBI), and Center for Biotechnology (CeBiTec), Bielefeld University, Germany
License: CC-BY 4.0
Copyrights: The authors retain unrestricted copyrights and publishing rights
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Wittler, Roland. General encoding of canonical k-mers. Peer Community Journal, Volume 3 (2023), article  no. e87. doi : 10.24072/pcjournal.323.

PCI peer reviews and recommendation, and links to data, scripts, code and supplementary information: 10.24072/pci.mcb.100188

Conflict of interest of the recommender and peer reviewers:
The recommender in charge of the evaluation of the article and the reviewers declared that they have no conflict of interest (as defined in the code of conduct of PCI) with the authors or with the content of the article.

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