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Class 4: Fast Sequence Alignment

Class 4: Fast Sequence Alignment. Alignment in Real Life. One of the major uses of alignments is to find sequences in a “database” Such collections contain massive number of sequences (order of 10 6 )

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Class 4: Fast Sequence Alignment

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  1. Class 4: Fast Sequence Alignment .

  2. Alignment in Real Life • One of the major uses of alignments is to find sequences in a “database” • Such collections contain massive number of sequences (order of 106) • Finding homologies in these databases with the standard dynamic programming can take too long • Example: • query protein : 232 AAs • NR protein DB: 2.7 million sequences; 748 million AAs • m*n = ~ 1.7 *1011cells !

  3. Heuristic Search • Instead, most searches rely on heuristic procedures • These are not guaranteed to find the best match • Sometimes, they will completely miss a high-scoring match • We now describe the main ideas used by some of these procedures • Actual implementations often contain additional tricks and hacks

  4. Basic Intuition • The main resource consuming factor in the standard DP is decision of where the gaps are. If there were no gaps, life was easy! • Almost all heuristic search procedures are based on the observation that real-life well-matching pairs of sequences often do contain long strings with gap-less matches. • These heuristics try to find significant local gap-less matches and then extend them.

  5. Banded DP • Suppose that we have two strings s[1..n] and t[1..m] such that nm • If the optimal global alignment of s and t has few gaps, then path of the alignment will be close to the diagonal s t

  6. Banded DP • To find such a path, it suffices to search in a diagonal region of the matrix • If the diagonal band has presumed width a, then the dynamic programming step takes O(an) • Much faster than O(n2) of standard DP in this case s a t

  7. Banded DP Problem (for local alignment): • If we know that t[i..j] matches the query s[p..q], then we can use banded DP to evaluate quality of the match • However, we do not know i,j,p,q ! • How do we select which sub-sequences to align using banded DP?

  8. FASTA Overview • Main idea: Find (fast!) “good” diagonals and extend them to complete matches • Suppose that we have a relatively long gap-less local match (diagonal): …AGCGCCATGGATTGAGCGA… …TGCGACATTGATCGACCTA… • Can we find “clues” that will let us find it quickly?

  9. s t Signature of a Match Assumption: good matches contain several “patches” of perfect matches AGCGCCATGGATTGAGCGA TGCGACATTGATCGACCTA

  10. FASTA • Given s and t, and a parameter k • Find all pairs (i,j) such that s[i..i+k] and t[j..j+k] match perfectly • Locate sets of pairs that are on the same diagonal by sorting according to i-j thus… • Locating diagonals that contain many close pairs. • This is faster than O(nm) ! s i i+k j j+k t

  11. FASTA • Extend the “best” diagonal matches to imperfect (yet ungapped) matches, compute alignment scores per diagonal. Pick the best-scoring matches. • Try to combine close diagonals to potential gapped matches, picking the best-scoring matches. • Finally, run banded DP on the regions containing these matches, resulting in several good candidate alignments. • Most applications of FASTA use very small k(2 for proteins, and 4-6 for DNA)

  12. BLAST Overview • FASTA drawback is its reliance on perfect matches • BLAST (Basic Local Alignment Search Tool)uses similar intuition, but relies on high scoringmatches rather than exact matches • Given parameters: length k, and threshold T • Two strings s and t of length k are a high scoring pair (HSP) if d(s,t) > T

  13. High-Scoring Pair • Given a query string s, BLAST construct all words w (“neighborhood words”), such that w is an HSP with a k-substring of s. • Note: not all k-mers have an HSP in s

  14. BLAST: phase 1 • Phase 1: compile a list of word pairs (k=3) • above threshold T • Example: for the following query: …FSGTWYA… (query word is in green) • A list of words (k=3) is: • FSG SGT GTW TWY WYA • YSG TGT ATW SWY WFA • FTG SVT GSW TWF WYS

  15. BLAST: phase 1 scores GTW 6,5,11 22 neighborhood ASW 6,1,11 18 word hits ATW 0,5,11 16 > threshold NTW 0,5,11 16 GTY 6,5,2 13 GNW 10 neighborhood GAW 9 word hits below threshold (T=11)

  16. BLAST: phase 2 • Search the database for perfect matches with neighborhoodwords. Those are “hits” for further alignment. • We can locate seed words in a large database in a single pass, given the database is properly preprocessed (using hashing techniques).

  17. s t Extending Potential Matches • Once a hit is found, BLAST attempts to find a local alignment that extends it. • Seeds on the same diagonal tend to be combined (as in FASTA)

  18. Two HSP diagonal • An improvement: look for 2 HSPs on close diagonals • Extend the alignment between them • Fewer extensions considered • There is a version of BLAST, involving gapped extensions. • Generally faster then FASTA, arguably better. s t

  19. Blast Variants • blastn (nucleotide BLAST) • blastp (protein BLAST) • tblastn (protein query, translated DB BLAST) • blastx (translated query, protein DB BLAST) • tblastx (translated query, translated DB BLAST) • bl2seq (pairwise alignment)

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