Molecular biology, gene regulation · BEGINNER LESSON
Transcription factor motif
A transcription factor motif is a short DNA sequence pattern that a transcription factor tends to recognize and bind. In practice, it is often represented as a score table or logo built from related DNA sequences that were observed at binding sites.
What it is and what is measured
A transcription factor (TF) motif is a short DNA sequence pattern associated with DNA binding by a TF, usually about 6–20 base pairs long. What is measured is the sequence preference of the TF: researchers compare many DNA segments that are bound, or suspected to be bound, and look for recurring sequence features that can be summarized as a motif. Motifs are used to annotate regulatory DNA regions and to predict where TF binding sites may occur in the genome. [0,1,2]
[1] Cross-platform motif discovery and benchmarking to explore binding specificities of poorly studied human transcription factors [2] Factorbook: an updated catalog of transcription factor motifs ... [3] Deep neural networks identify sequence context features predictive ...
How motifs are represented and found
A common representation is the position weight matrix (PWM), also called a position-specific scoring matrix (PSSM). A PWM stores how favored each DNA letter is at each position in the motif, and it assumes neighboring positions contribute independently to binding energy. Motifs are often derived from collections of related sequences found in experiments such as chromatin immunoprecipitation sequencing (ChIP-seq) or high-throughput systematic evolution of ligands by exponential enrichment (HT-SELEX). Some tools instead summarize motifs as aligned short sequence words, or k-mers, and then score new sequences against that pattern. [0,1,2]
[1] Cross-platform motif discovery and benchmarking to explore binding specificities of poorly studied human transcription factors [2] Factorbook: an updated catalog of transcription factor motifs ... [3] Deep neural networks identify sequence context features predictive ...
Why motif instances do not always mean binding
A motif can appear many times in the genome, but only some occurrences are actually bound in a given cell type. Whether a motif instance is bound depends not only on the sequence pattern itself but also on context features such as chromatin accessibility, nucleosome positioning, nearby cooperating or competing factors, local GC composition, and DNA shape or other local sequence context. This means a motif is a useful clue, but not a complete explanation of binding inside living cells. [2,1]
[3] Deep neural networks identify sequence context features predictive ... [2] Factorbook: an updated catalog of transcription factor motifs ...
Limitations
PWMs are useful and widely used, but they cannot capture dependencies between positions in the motif. That limitation matters because some TFs show interactions among positions that help distinguish bound from unbound sequences. Another limitation is that motif discovery tools can return more than one candidate motif, and the top result is not always the best in every setting; performance can vary by dataset, experiment type, and software. [0,2]
[1] Cross-platform motif discovery and benchmarking to explore binding specificities of poorly studied human transcription factors [3] Deep neural networks identify sequence context features predictive ...
SEE THE IDEA
How a transcription factor motif relates to binding
A stretch of letters in the genome.
A short recurring sequence preference, often summarized by a PWM.
Chromatin accessibility and nearby factors affect whether binding happens.
The TF is actually found bound at that site in a cell type.
DNA sequence contains a motif pattern; cell context helps decide whether that motif becomes an observed binding site.
A motif is a sequence clue, while binding in cells also depends on local genomic and cellular context.
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.