<p>Your confusion is understandable, since it isn't explained in the <a href="https://www.ncbi.nlm.nih.gov/refseq/targetedloci/16S_process" rel="nofollow noreferrer">NCBI description</a> of the database, but most of the sequences in the database are derived from DNA that encodes the 16S rRNA <em>gene</em>, not from the rRNA itself. In some cases, sequences are derived from cDNA that is synthesized using the actual rRNA as a template, but it's still DNA sequencing. Direct RNA sequencing is <a href="https://www.nature.com/articles/nmeth.4577" rel="nofollow noreferrer">extremely uncommon</a>. Depending on the cDNA sequencing protocol (first strand vs. second strand sequencing), the transcript sequence might be the reverse-complement of the 16S rDNA, but the only difference that you would see is that the numbering of the bases would be reversed. If you are sequencing rDNA from organelles, there may also be a few bases that are different, due to <a href="https://en.wikipedia.org/wiki/RNA_editing" rel="nofollow noreferrer">RNA editing</a>.</p>

CC BY-SA 3.0
2018-04-09T09:21:13.040Z
2018-04-09T09:21:13.040Z
heathobrien
<p>I have 16S DNA sequences from diversity sequencing, and want to query these against NCBI's 16S rRNA database.  If I ultimately want to get taxids for the species that my sequences have, does it make a difference that my sequences are 16S DNA and not RNA?  I am using Blast+.  </p>

<p>I imagine this is how analysis of 16S sequences normally goes (get 16S DNA sequences, query them against a 16S DB), but does anyone know if Blast+ or the 16S rRNA DB itself allow for DNA queries?  </p>

<p>In other words, if I were to blast a sequence of 16S DNA and its exact transcript RNA, would I get the same results?</p>

<ngs><blast>
Is it OK to use Blast+ to query NCBI's 16s rRNA database with 16s DNA sequences?
# Is it OK to use Blast+ to query NCBI's 16s rRNA database with 16s DNA sequences?

(Asked by: username_0 on 2018-04-07)

I have 16S DNA sequences from diversity sequencing, and want to query these against NCBI's 16S rRNA database. If I ultimately want to get taxids for the species that my sequences have, does it make a difference that my sequences are 16S DNA and not RNA? I am using Blast+.

I imagine this is how analysis of 16S sequences normally goes (get 16S DNA sequences, query them against a 16S DB), but does anyone know if Blast+ or the 16S rRNA DB itself allow for DNA queries?

In other words, if I were to blast a sequence of 16S DNA and its exact transcript RNA, would I get the same results?

**Tags:** ngs, blast

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## 1 answers

### Answer by username_1 on 2018-04-09. Score: 2 | Accepted answer

Your confusion is understandable, since it isn't explained in the [NCBI description](https://www.ncbi.nlm.nih.gov/refseq/targetedloci/16S_process) of the database, but most of the sequences in the database are derived from DNA that encodes the 16S rRNA *gene*, not from the rRNA itself. In some cases, sequences are derived from cDNA that is synthesized using the actual rRNA as a template, but it's still DNA sequencing. Direct RNA sequencing is [extremely uncommon](https://www.nature.com/articles/nmeth.4577). Depending on the cDNA sequencing protocol (first strand vs. second strand sequencing), the transcript sequence might be the reverse-complement of the 16S rDNA, but the only difference that you would see is that the numbering of the bases would be reversed. If you are sequencing rDNA from organelles, there may also be a few bases that are different, due to [RNA editing](https://en.wikipedia.org/wiki/RNA_editing).


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