Automated Preparation of Long Insert PacBio Libraries

The GRC sequences thousands of microbial samples each year. The high throughput of our sequencers and the small genome sizes of these samples means that a lot of libraries are needed to keep our sequencers running at full capacity. Automation of library preps is key to keeping instruments busy, reducing error, and maximizing the productivity of our lab staff.

We have used automation to prepare large batches of libraries for Illumina sequencing for several years. Earlier this year, we began testing preparation of long-insert libraries for our PacBio RS II platform.

Our Biomek FXP (BeckmanCoulter) is a Dual Multi-channel Span-8 with an integrated thermal cycler. The combination of a 96-channel pipetting head and Span-8 pipetting head (which has eight independently controlled channels) allows for the preparation of up to 96 libraries at a time and the minimization of master mix dead volumes.

A script for preparing SMRTbell libraries developed by Todd Hartley at NCI/SAIC-Frederick was downloaded from Pacific Biosciences SMRT Community. We modified to the protocol to accommodate the specific tips and reagents tubes our lab uses, and to optimize reaction mixing.

 

Deck Layout

PB_Biomek_Deck

Prior to being loaded on the Biomek, samples are sheared with g-TUBEs (Covaris, Woburn, MA), targeting an average fragment size of 20kb. Master mixes for each reaction are prepared and placed in the robot. The following steps are performed by the Biomek:

  • DNA damage repair
  • End Repair
  • Ampure clean up
  • SMRTbell Ligation
  • Exonuclease
  • Ampure clean up

Once the run is complete, the libraries are removed and size-selection is performed using the BluePippin (Sage Science, Beverly, MA).

  manual preps robotic preps
Number of libraries (n) 91 27
Average input amount (ng of sheared gDNA) 4945 4951
Average library size (bp) 18507 18910
Average library concentration (nM) 5.1 3.5
Average recovery (ng) 902.8 707.4
Average recovery (%) 18.7 14.2

Above are data comparing libraries prepped manually and on the Biomek, from March 2014 to date. While the yields from the Biomek preps are slightly lower than manual preps, the yield is comparable and sufficient for sequencing multiple SMRT cells per library.

For more information on our full range of sequencing and analysis services, visit our Laboratory Services and Analysis Services pages. Please contact us if you have any questions.

 

Options When Starting Material is Limiting

Sometimes it is not possible to come up with the amount of DNA or RNA required for a standard Illumina library prep. We are frequently asked what the options are when there is just not enough sample available.

There are several kits on the market now that allow Illumina libraries to be prepared from minimal amounts of starting material. We have processed clinical samples, metagenomic samples, and samples from FFPE tissues that yielded extremely low amounts of RNA or DNA.

For RNA samples, we have generated linearly-amplified cDNA with the Nugen Ovation v2 kit. An advantage of this kit is that the amplification of rRNA is somewhat suppressed, increasing the percentage of usable data. Starting with sub-nanogram amounts of RNA, we are able to generate micrograms of cDNA. We’ve tested various library preparation methods with the amplified cDNA, and we have found that the Illumina TruSeq prep to work the best for us.

The Illumina Nextera system is an option available when DNA amounts are limiting. The Nextera XT DNA Sample Prep Kit requires exactly 1 ng of input material (best for plasmids or small genomes), and the Nextera kit DNA Sample Prep Kit requires exactly 50 ng of DNA. The library fragmentation is accomplished via transposon insertion events. We skip the normalization/denaturation portion of the protocol, and determine the quality and quantity of the libraries following our standard procedures. We have found that the library sizes tend to vary, and can be much wider than our traditional Illumina DNA libraries, but this is still a great option when there is very little material available.

Contact us if you have questions or would like additional information.