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Plasmids containing T7-ccdB

We constructed plasmid pSB6A1-T7ccdB for use in CIChE. Through this plasmid, pressure can be put on the CIChE strains simply by adding IPTG.

We also purified plasmids p5SpFRT-T7ccdB, p10SpFRT-T7ccdB, p20SpFRT-T7ccdB from strains we obtained from Inbio. These plasmids have different copy numbers and they too can be used in chromosomal evolution.

Having plasmids with different copy numbers at our disposal, we can test CIChE with different degrees of toxin-pressure.

Strains with ccdA-gfp construct

The following strain containing the construct to be duplicated (ccdA-gfp) was constructed (KI experiment 1):

We also dispose of a backup version of this strain, constructed by Inbio in case our own experiment failed (called MDM.Cm). We included this second version in our further experiments as to test possible differences between the two versions.

CIChE Strains

The plasmids and strains mentioned above were used in transformations (experiment 3) to obtain strains containing all elements necessary to perform chromosomal evolution. The following strains were constructed:

UV Test

Phage transduction was used to perform the deletion of the recA gene after chromosomal evolution. To check whether the gene was successfully knocked out, an UV test was developed. The UV light puts a certain amount of stress on the bacterial cells. In respons to this stress, an SOS pathway is triggered in which recA plays an important role. Cells in which the recA gene is deleted will not survive on the UV-treated plate. Colonies in which recA was successfully deleted still can be found on a non-treated backup plate.

RESULT:

recA positive strain grows on both parts (UV and non-UV) recA negative strain 10’’ – no visible differences 15’’ – still some colonies on UV-part of the plate 20’’ – one colony visible on UV-part of the plate 30'' – no growth on UV-part of the plate

CONCLUSION
With a 30’’ UV exposure, the cells in which recA is successfully deleted will not survive on the UV-treated plate.

CIChE

Why are we having so much trouble?

For the construction of our plasmid containing the ccdB gene we chose to work with a T7 promoter for the ccdB gene as to avoid leaky expression. Taking into account the troubles we had with the construction of the plasmids with the T7-ccdB insert we can almost certainly conclude that this promoter does still show significant leaky expression, as the toxin gene obviously hindered the cells although the promoter was not induced. Often we obtained no colonies after transformation and if we did, there were mutations in the ccdB gene. We performed CIChE with the plasmid we constructed ourselves (pSB6A1-T7ccdB) as well as with the original plasmids from which we constructed this part: p5SpFRT-T7ccdB, p10SpFRT-T7ccdB and p20SpFRT-T7ccdB. According to the GFP measurements we performed, none of these resulted in duplication of the ccdA-gfp construct in the genome. We presume that this is caused by the malfunction of CcdB caused by mutations. But how is it possible that there’s so many of these mutations?

It is known from previous research that the part essential for CcdB’s toxic activity is situated at the last three amino acids (Trp-Gly-Ile). Interaction between CcdB and the DNA gyrase A subunit (GyrA) is situated at Trp99 from CcdB and Arg462 of GyrA59 (the 59 kDa amino-terminal breaking-rejoining domain of GyrA). (Source: Dao-Thi et al., 2005) Random mutations in the final part of the ccdB gene affecting this toxic site will consequently provide these cell with a great growth advantage over cells with intact ccdB genes. We assume this is a possible reason why mutation frequency of the ccdB gene seems to be so high in our project.

Furthermore, in the absence of its antidote CcdA, CcdB causes reduced DNA synthesis, activation of the SOS pathway, cell filamentation and eventually cell death. (Couturier et al., 1998) One of the aspects of this SOS pathway is the enhancing of capacity of mutagenesis, such as point and frameshift mutations. So if for example there is leaky expression of the promoter and CcdB is being formed without a sufficient amount of CcdA present, SOS pathway will be activated causing higher mutation frequencies. We postulate that these mutations can also affect the ccdB gene itself and thus restoring ‘normal’ cell life. In combination with what we discussed in the previous paragraph this could explain the amount of mutations we had to deal with over the course of our project.

References of these last two paragraphs:
Dao-Thi, M.H. et al. Molecular basis of gyrase poisoning by the addiction toxin CcdB. Journal of Molecular Biology 348, 1091-1102 (2005).
Couturier, M., Bahassi, E. & Van Melderen, L. Bacterial death by DNA gyrase poisoning. Trends in Microbiology 6, 269-275 (1998).

Randomly we chose a strain at a point during the CIChE experiment (8 + pSB6A1-T7ccdB, 2 mM IPTG day 2, Step) and let the plasmid get sequenced. When we compared the ccdB sequence to the native one we saw our plasmid had a deletion of a cytosine residue at position 297 (out of a total of 306 base pairs). This causes a frameshift, resulting in a prolonged protein of 154 amino acids instead of the native structure with 102 amino acids. (Note: CcdB normally contains 101 amino acids, but the sequence we used has an extra Alanine after the start codon for cloning purposes.)

What can possibly be doon about it?

Assemble T7-ccdB plasmid inside CcdB-survival strains

When assembling the plasmid containing T7-ccdB in cells resistant to CcdB, mutations as a result of stress are less likely to occur at this stage. We experienced difficulties cloning the T7-ccdB construct in our plasmids probably because of leaky expression of the T7-promoter.

Use Arabinose promoter with glucose repressor