BLAT

BLAT aligns DNA and protein sequences rapidly to identify genomic locations and exon-intron structures for mRNA/DNA and cross-species protein alignments, particularly in vertebrate genomes.

Key Features:

• Nonoverlapping K-mer genome index: Uses an index comprising all nonoverlapping K-mers from the genome that is computed once per genome assembly and stored in RAM.
• Index-based homology detection: Leverages the precomputed K-mer index to identify genomic regions likely homologous to the query sequence.
• Precise regional alignment: Performs alignments between identified homologous regions to generate accurate local matches.
• Exon stitching into gene alignments: Stitches aligned segments, often exons, into larger contiguous alignments that typically represent entire genes.
• Exon refinement and splice-site adjustment: Revisits smaller internal exons potentially missed initially and adjusts boundaries of large gaps where canonical splice sites are present.
• Parameter optimization for speed/sensitivity: Explores K-mer sizes, mismatch schemes, and the number of required index matches to balance speed and sensitivity.
• Performance: Achieves substantial speed improvements (approximately 500× for mRNA/DNA alignments and 50× for protein alignments) under typical sensitivity settings for vertebrate sequence comparisons.

Scientific Applications:

• mRNA-to-genome alignment: Determining exon-intron structure and mapping mRNA sequences to genomic loci.
• Cross-species protein alignment: Identifying homologous protein regions across species for comparative analyses.
• Genome-wide analyses in vertebrates: Large-scale mapping and annotation across genome assemblies using precomputed indices.
• High-throughput alignment tasks: Enabling rapid alignment of many queries due to the in-memory, precomputed index approach.

Methodology:

Build a genome index of all nonoverlapping K-mers stored in RAM (computed once per assembly), use the index to locate candidate homologous regions, perform alignments on those regions, stitch aligned segments (often exons) into contiguous gene alignments, and iteratively refine by revisiting small internal exons and adjusting large-gap boundaries based on canonical splice sites while tuning K-mer size, mismatch schemes, and required index-match counts.