Team:UC Davis/notebook/c0051debug.html

Characterization of BBa_C0051, one of the most widely used and studied parts

Bba_C0051 cI-Lamba, the promoter in disguise During the construction of one of our larger pieces of the spatial oscillator, the ON switch, we stumbled upon an unexpected phenotype.

The RBS-RFP in this picture is actually RBS-RFP-F1610, but for simplicity purposes and purposes of explaining what's going on, we have illustrated it as RBS-RFP

The Problem

Our scanned image of our plate with colonies containing RBS-C0051 ligated to RBS-RFP-F1610. Click above for the full-screen image.

Our intermediate ligation of RBS-C0051 to RBS-RFP-F1610 produced a red phenotype but it has no promoter! Since Bba_C0051 is a highly studied and widely used part in the registry, it presents a huge problem not only in the context of the spatial oscillator but also in the 76 parts that utilize it throughout the registry. This exciting new find causes many problems in our design and must cause malfunctions in a huge number of parts entered and used in the registry.

In Silico Promoter Scanning of C0051 Coding Region

• While the cI lambda phage repressor is one of the best studied and most used proteins in synthetic biology we wanted to start our search for potential promoters by asking whether or not we could detect any putative promoters in the coding region of cI by in silico methods. We tried a number of on-line tools for automated promoter detection but did not come up with any meaningful results. Next we contacted Drew Endy at Stanford to tap into his knowledgebase of phage biology. He forwarded this request to Thomas Schneider at the NIH who used the Delia programs to search for sigma70 binding sites. Surprisingly, a few potential "good" scoring candidates were found. Armed with this data, we decided to see if we could experimentally confirm one of these sites as a potential transcript start site using the technique called 5'-RACE. Initial results follow.

The Investigation

We began by sequencing the DNA of the red colonies from the ligation of RBS-C0051 to RBS-RFP-F1610 in order to rule out the presence of any promoter or mutation. The sequence analysis revealed that we did indeed have the correct sequence. Results of our C0051 w/ LVA tag sequence.. Further investigation into Bba_C0051 showed that the part not only contained the coding region, cI, but it is also followed by an LVA degradation tag and a barcode. When looking further into the RBS (Bba_E1010), we found that it also contained an LVA degradation tag and a barcode after the RFP coding region. We then conducted a number of experiments in order to determine which part or combination of parts contained in the construct caused this unwanted promoter-like behavior. After determining the cause of the unwanted transcription, we continued to conduct a number of experiments in order to measure the results.

A Starting Point

Our scanned image of our plate with colonies containing RBS-RFP-F1610. No abnormalities here! Click to enlarge.

The previous step in the construction of the on switch was RBS-RFP-F1610. This construct clearly exhibited a white phenotype. No abnormalities here!

Test #1

To determine whether Bba_C0051 played a role in the unexpected transcription, we ligated RBS-RFP to RBS-RFP in order to see if transcription occurred. The construct exhibited a white phenotype. Thus, we concluded that transcription did not start in any part of the RFP. This helped to narrow our focus down to the possibility of the cI coding region in Bba_C0051 or a possible artifact in the plasmid as the cause for the unwanted transcription. After the results were in, we diligently sent them out for sequencing so that we would be sure that our results were accurately reflecting the parts that we believed we were analyzing.

Test #2

Our scanned image of our plate with colonies with C0051 into Chris Anderson's promoter screening plasmid. Click to enlarge.

Next, we sought to determine whether the presence of an artifact in the plasmid might be the cause or contribute to the promoter function. We ligated RBS-C0051 to Chris Anderson's Promoter Screening Plasmid (Bba_J23006) By changing the context in which the parts were placed, we were able to removing the possibility of the artifact causing transcription Even after placing this part into a new context, we obtained red colonies! Therefore, we know that the transcription initiation has something to do with the Bba_C0051 region. Sequenced? Yes! For sequencing data, click here.

Test #3

Our scanned image of our plate with colonies containing RBS-RFP in Chris Anderson's PSP. Click above for the full-screen image.

Now, although the presence of an artifact from the original plasmid has been determined to be negligible, it is also important to verify the same for Chris Anderson's Promoter Screening Plasmid (Bba_J23006) We ligated RBS-RFP into Chris Anderson's Promoter Screening Plasmid (Bba_J23006) A white phenotype was expressed. Therefore, it is clear that we must focus down on the Bba_C0051 part in order to determine where a promoter function is being expressed. Once again, of course this part was sequenced!

5' Race PCR

In order to locate the transcript start site of "mystery C0051" promoter, we conducted a 5'-RACE experiment on three cell lines. (1) C0051-PSP (C0051 in a promoter screening plasmid - red cells), (2) E1010-PSP (RFP in promoter screening plasmid - white cells) and (3) DH5alpha cells. Using RFP specific primers, we were unable to generate any cDNA from either constructs 2 or 3 - our negative controls. We were able to retrieve transcript from the C0051-PSP strain indicating that RFP was indeed transcribed. When we sequenced the resulting cDNA (see figure below) we noticed that the poly-A tail (added during sample prep for amplification) was located just upstream of the scar between the barcode and the ribosome binding site (Bba_B0034). Therefore, our initial 5'-RACE experiment indicates that the transcript likely initiates within the bar code region rather than within the cI coding region. A second sample/clone verified this result

Figure legend: Sequencing chromatogram of 5'-RACE experiment.

As shown, the transcription that we are seeing clearly starts at the beginning of the barcode region So, what does this all mean?

Test #4

Modified C0051 in PSP exhibited white phenotype. Click above for the full-screen image.

From all the test so far, it is clear that a certain interaction of the cI coding region and the barcode are acting together to produce some kind of promoter function. In order to experimentally provide data to prove this point, we removed the barcode from the RBS-C0051 construct and placed it into Chris Anderson's Promoter Screening Plasmid (Bba_J23006). Surely enough, the phenotype was white! We finally narrowed it down and were able to create a part in which the cI lambda coding region can be utilized without creating an unwanted and detrimental promoter function. This part was sequenced and submitted!

The Fix

More to Come!