BLAST masking

Question

[timothy.kirkwood]

Hello,

If I want to apply the BlastP -db_soft_mask flag using BLAST+ v 2.10.1, am I correct in thinking that I will need to build a masked database first (i.e. I can't apply the flag to a standard DB made using makeblastdb)? Also, this link says

Database masking has two modes. The first is known as "soft-masking", and BLAST uses the database mask only during the (initial) word-finding phase of BLAST. The second is known as "hard-masking", and BLAST uses the database mask during all phases of the search.

To clarify, does this mean that when a hit is found in the soft-masked database, the hit sequence is 'demasked' and analysed for HSPs in full?

Thanks!

[Mensur Dlakic]

You do not need to do anything different when building a BLAST database - masking is done on the fly.

[timothy.kirkwood]

Thanks for your reply! The flag needs a string argument from what I can tell:

-db_soft_mask <String>
   Filtering algorithm ID to apply to the BLAST database as soft masking
    * Incompatible with:  db_hard_mask, subject, subject_loc

I think the string is the masking algorithm ID that accompanies a masked database (if you make one), from here:

To enable database masking during a BLAST search, we use the –info parameter of blastdbcmd to discover the masking Algorithm ID.

The link is explicitly using a masked database that was made here.

What would the string be if you didn't make a masked DB?

[Mensur Dlakic]

BLAST used to have dust and seg algorithms for masking DNA and protein sequences, respectively. For those two there was no need to change anything during the database creation process. Now it seems that -dust and -db_soft_mask are separate options. My apologies, I should have looked into this more carefully before commenting.

[timothy.kirkwood]

No worries - I think dust might not be included in the BLASTP flags now, although seg still is (for queries only). Dust is still included as a program in the BLAST+ package though, potentially supporting it's intended use being in masked database creation before the run!