Beta-Lactamase

What is Beta-lactamase?

Beta-lactamase (BLA) is an enzyme encoded by the ampicillin resistant gene (ampr) frequently present in plasmids for selection. Structurally, beta-lactamase is a 29-kDa monomeric enzyme (figure 1). Its enzymatic activity provides resistance to beta-lactam antibiotics such as cephamysin, carbapenems and penicillin through hydrolysis of the β-lactam ring, a structure shared by these antibiotics (Qureshi, 2007).

Many advantages come from working with beta-lactamase. It shows high catalytic efficiency and simple kinetics. Also, no orthologs of BLA are known to be encoded by eukaryotic cells and no toxicity was identified making this protein very useful in studies involved eukaryotes (Qureshi, 2007). Beta-lactamase has been used to track pathogens in infected murine models (Kong et. al, 2010). However, in addition to its application in eukaryotic cells, beta-lactamase efficiently cleaves a wide variety of substrates but its versatility goes beyond that; BLA preserves its activity even when fused to heterologous protein (Moore et. al, 1997). This feature, in particular, makes beta-lactamase a potential tool for assemble of synthetic constructs. We retrieved the BLA gene from the backbone of the pSB1A3 plasmid and added a His-tag to it. We also fused a flexible glycine linker (BBa_K157013) to the N-terminus of BLA so we could later connected to our detector. More details on how these procedures were done can be found at our Reporter Journal.

How is Beta-lactamase used as a Reporter?

Beta-lactamase, in the presence of different substrates, can give various outputs. It can produce a fluorogenic output in the presence of a cephalosporin derivative (CCF2/AM) and BLA enzymatic activity can be detected by a fluorometer (Remy et al., 2007).

Besides fluorescence assays, beta-lactamase can also be used to obtain colorimetric outputs by breaking down synthetic compounds such as nitrocefin (figure 2). The color change goes from red to yellow (Remy et al., 2007). Colorimetric assays can also be done with penicillin G as the substrate, which, in addition, gives a pH output that can be detected with pH indicators (Li et al., 2008).

BLA can also be split apart in to two halves for protein complementation assays where each half is linked to the two proteins that might be interacting. If they are, then when they interact, the two halves are able to fold into their correct structure and give an output (Wehrman et al., 2002).

Therefore, this enzyme gives a lot of flexibility in terms of how it is used in a system, which makes it an useful reporter to add to the Parts Registry and characterize it.

How does Beta-lactamase fit in our Biosensor?

In the system we are designing, beta-lactamase serves as a reporter as much as prussian blue ferritin. If enterohemorrhagic DNA is present in the sample, the immobilized TALE will capture it in solution. A mobile TALE, which is linked to BLA, will bind to the target DNA, a sequence in the Stx2. The strip is them washed to remove unbound TALEs and a substrate is added to give the color output.

For characterization purposes, we are working on testing our constructs with the two colorimetric substrates mentioned above: nitrocefin and penicillin G. So far we have demonstrated that our mobile TALE linked to beta-lactamase constructional retained it enzymatic activity. We performed an Ampicillin Survival Assay and bacteria susceptible to ampicillin was able to grow in the presence of our construct.