PSI-BLAST for Remote Protein Homolog Detection
Identifying distantly related protein homologs is crucial for understanding protein function and evolution. This involves finding proteins that share a common ancestor but may have diverged significantly over time.
Analysis of Homology Search Programs
Different BLAST programs serve distinct purposes in sequence analysis:
- BLASTN: Compares nucleotide sequences to nucleotide databases. It is not used for finding protein homologs.
- BLASTX: Translates a nucleotide query sequence into proteins (in all six reading frames) and searches against protein databases. It can find homologs but is less sensitive for very distant relationships compared to PSI-BLAST.
- TBLASTX: Compares a nucleotide query sequence (translated in all six reading frames) against a nucleotide database (also translated in all six reading frames). It's computationally intensive and generally less preferred than other methods for finding distant protein homologs.
- PSI-BLAST (Position-Specific Iterated BLAST): This program is specifically designed to find more distantly related homologs.
Why PSI-BLAST is Key for Distant Homologs
PSI-BLAST employs an iterative approach:
- It performs an initial BLAST search.
- It uses significant matches from the first search to create a Position-Specific Scoring Matrix (PSSM). This matrix captures conserved positions within the potential homolog family.
- It re-iterates the search using the PSSM, which allows it to detect weaker, more distant similarities that might be missed by a standard BLAST search.
This iterative refinement makes PSI-BLAST the most effective tool among the options for identifying distantly related protein homologs.
Important Questions from Methods in Microbiology
- Which one of the following is used for global alignment of two protein sequences?
- Correctly match the following Bioinformatic tool/Database with its respective Utility.
Bioinformatic tool/Database Utility P. BLAST 1. Database for 3D protein structures Q. Bowtie 2. Tool to identify similarity of a query sequence to existing sequences available in databanks R. AlphaFold 3. Tool to align short read DNA sequences obtained from Next-generation sequencing to a reference genome S. PDB 4. AI tool to predict protein structures - Which of the following is NOT used for generating an optimal alignment of two nucleotide sequences?
- Which one of the following BLAST search programs is used to identify homologs of a genomic DNA query in a protein sequence database?
Consider the following alignment of two DNA sequences:
AGTAAC
AA--AC
Assuming an affine gap scoring scheme of an identity matrix for substitution, a gap initiation penalty of 1 and a gap extension penalty of 0.1, the score of the alignment is (up to one decimal place)_______.
