Psiblast

psiblast

Quick Start

• Command: psiblast
• Local executable: /home/vimalinx/miniforge3/envs/bio/bin/psiblast
• Version: 2.17.0+
• Full reference: See references/help.md for complete options and usage details

When To Use This Tool

• Detect distant protein homologs that plain blastp may miss.
• Build a PSSM for reuse in later searches.
• Restart from an existing PSSM or MSA checkpoint.
• Use only when you are prepared to monitor profile drift across iterations.

Common Patterns

# 1) Iterative distant-homology search with saved PSSM
psiblast \
  -query query.fa \
  -db prot_db \
  -num_iterations 5 \
  -evalue 1e-3 \
  -inclusion_ethresh 1e-3 \
  -out_pssm query.pssm \
  -outfmt 7

# 2) Save an ASCII PSSM for inspection or downstream tools
psiblast \
  -query query.fa \
  -db prot_db \
  -num_iterations 3 \
  -out_ascii_pssm query.ascii.pssm

# 3) Restart from an existing checkpoint
psiblast \
  -in_pssm query.pssm \
  -db prot_db \
  -num_iterations 2 \
  -outfmt 6

Recommended Workflow

1. Start from a high-confidence protein query or curated alignment.
2. Set -inclusion_ethresh conservatively so poor hits do not contaminate the profile.
3. Inspect each iteration's accepted hits before trusting convergence.
4. Save the final PSSM if the profile will be reused or compared later.

Guardrails

• Input must be protein sequence, protein MSA, or protein PSSM, never nucleotide query sequence.
• More iterations are not always better; false positives can poison the profile early.
• -evalue controls reporting, while -inclusion_ethresh controls what enters the next-round model.
• Save checkpoints (-out_pssm, -out_ascii_pssm) if the iterative result matters scientifically.