Webtag: a new web tool providing tags/anchors for RT-PCR experiments with prokaryotes
© Pinto et al; licensee BioMed Central Ltd. 2007
Received: 01 June 2007
Accepted: 25 October 2007
Published: 25 October 2007
Webtag is a tool providing oligonucleotide sequences (usually called tags or anchors) that are absent from a specified genome. These tags/anchors can be appended to gene specific primers for reverse transcriptase polymerase chain reaction experiments, circumventing genomic DNA contamination.
The use of a relational database, in conjunction with a series of scripts written in PHP and Perl, allows the user to rapidly obtain tags that are: 1) suitable for a specific organism, and 2) compatible with other oligonucleotides to be used in the experimental procedures.
This new web tool allows scientists to easily and rapidly obtain suitable tags for RT-PCR experiments, and is available at http://www.egs.uu.se/software/webtag/.
In order to better understand different aspects of metabolism it is important to study the underlying transcriptional profile. A key factor to assemble such profiles is the ability to obtain good gene expression data. For that purpose, reverse transcription polymerase chain reaction (RT-PCR)  became the method of choice more than a decade ago [2, 3].
RT-PCR allows exponential amplification of even a very low copy number mRNA. Because of its high sensitivity, the process is very vulnerable to DNA contamination.
Unfortunately, no RNA extraction method can guarantee the absolute absence of DNA in any given sample, ultimately leading to amplification, during PCR, of both cDNA and contaminating genomic DNA [4–11].
The effects of DNA contamination can be overcome using techniques like oligo d(A) selection, intron spanning primer design, DNase I treatment or restriction endonuclease digestion [7, 8, 11]. However, these procedures can be time consuming, expensive and contribute to RNA degradation. Moreover, in the particular case of prokaryotes, oligo d(A) selection and intron spanning primer design are not applicable solutions.
Due to the constant growth in available genomic data, tags that were before considered adequate, or are even part of commercially available kits, produce significant alignments for many microorganisms when BLASTed (data not shown). Tagenerator  was our first effort to give the molecular biologist the possibility to associate to the potential of the above described methods with the use of genome-absent tags. The tool proved useful, but the software requirements (Perl, BioPerl  and BLAST ) and the long runtime for bigger genomes made it somewhat unpractical for some users. In order to improve the ease of use we decided to create Webtag – a web based tool that, while having the same goal as Tagenerator, would be based on improved algorithms for tag/anchor generation, much faster runtime and the possibility to handle batch runs.
Basic components of the Webtag tool
The web tool consists of two components: 1) a MySQL relational database and 2) a web interface implemented in PHP, running on an Apache web server and using Perl scripts for batch processing. Prior to building the database, Perl scripts were also used in the generation of tag sequences, genome adequacy evaluation (BLAST) , melting temperature calculation and likelihood of secondary structure formation [15, 16].
Generation of genome adequate tags
Since specific, high yield, PCR can only be achieved using carefully designed primers, the following tag construction parameters were considered and implemented: tag length, melting temperature, GC content, absence of repeats and absence of secondary structures.
After full assembly, all sequences were analyzed for GC content (40% to 60%), correct melting temperature (above 52°C) [15, 16] and secondary structure formation (maximum free energy must be above -4 kcal/mol for dimer formation, and above -3 kcal/mol for hairpin formation) . This process resulted in the generation of more than half a million unique tags.
All tags are then tested for genome adequacy, against each prokaryotic genomic sequence found at the NCBI FTP server . For this purpose BLAST settings are defined as length 7 and E value 10. With such settings, even statistically poor hits resulted in tag rejection.
Finally, suitable tags were then integrated in the relational database.
If needed, it is possible to submit batch jobs to Webtag. When using this feature a group of sense primers must be supplied by the user (in some cases, reverse transcription gene specific primers can be specified along with the sense primers). Three types of batch jobs can be run, depending on the user's needs:
Type 1 – returns one tag for each submitted sense primer or a pair consisting of sense primer and reverse transcription gene specific primer.
Type 2 – after returning one tag for a sense primer (or a pair consisting of sense primer and reverse transcription gene specific primer), Webtag checks whether the tag may be compatible with other submitted queries. In this way, the number of needed tags is reduced.
Type 3 – only one tag will be returned, compatible with a list of sense primers submitted by the user (RT gene specific primers are not considered). The average melting temperature is calculated and used to select a set of compatible tags. These tags are then checked for secondary structure formation against all sense primers, and the first tag to have all dimerization free energies above -6 kcal/mol is output as a suitable tag.
Results and Discussion
Molecular biology methods
RNA extractions from Nostoc PCC73102 were carried-out using TRI Reagent (Molecular Research Center, Inc – USA), based on protocol given by the supplier. All RT-PCR reactions were performed in a two-step fashion using the RevertAid First Strand cDNA Synthesis Kit (Fermentas, Lithuania) and 2× PCR Master Mix (Fermentas, Lithuania), according to manufacturer's instructions. Between these two steps cDNA was cleaned using MSB Spin PCRapace kit (Invitek, Germany), following manufacturers' protocol.
Validation of new tag generation algorithm
The new method to generate tags resulted in higher sequence variation, since it does not limit the 3' and 5' ends to a pre-defined list . As a consequence, more than half a million unique sequences were found possessing good priming.
Primers used in RT-PCR and generated PCR product sizes (in base pairs). During PCR the tag used as antisense primer was CAACAGACGCACGACGCAGCAGAC (bold in the tagged RT primer sequences).
Tagged RT primer
Webtag database content and output
At the moment, the database holds tags suitable for 410 prokaryotic strains, downloaded from the NCBI FTP server . The database will be frequently revised to include additions to NCBI.
As shown in Figure 3, the final results are returned in a simple format that facilitates primer ordering and experimental procedure planning.
Webtag is a unique web service allowing the user to rapidly obtain tags that are: 1) suitable for a specific organism, and 2) compatible with other oligonucleotides to be used in the experimental procedures.
Availability and requirements
Project name: Webtag
Project web page: http://www.egs.uu.se/software/webtag/
Operating system(s): platform independent
Other requirements: web browser and internet connection
License: free for academic use
Restrictions to use by non-academics: license needed
List of abbreviations used
basic local alignment search tool.
complementary deoxyribonucleic acid.
genomic deoxyribonucleic acid.
messenger ribonucleic acid.
National Center for Biotechnology Information.
polymerase chain reaction.
reverse transcription-polymerase chain reaction.
This work was financially supported by the Knut and Alice Wallenberg Foundation, the Swedish Energy Agency, the Nordic Energy Research Program (project BioHydrogen), and the EU/NEST Projects SOLAR-H (contract #516510) and BioModularH2 (contract #043340). Håkan Svensson received financial support from the Helge Ax:son Johnsons Stiftelse. Ola Lundström (Department of Evolution, Genomics and Systematics, Uppsala University) is acknowledged for his contribution to the layout and artwork of the webpage. We would like to acknowledge Åsa Söderberg (Department of Photochemistry and Molecular Science, The Ångström Laboratories, Uppsala University) for her input during the development and testing of this tool.
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