AldB Gene (Detects Ethanol)

The aldB gene, which codes for a functional aldehyde dehydrogenase is induced by ethanol, a VOC that exists in higher concentrations in the breath of lung cancer patients. In E. Coli, the metabolism of ethanol consists of its transformation into an aldehyde and then into a carboxylic acid. The latter product then reacts with the g protein receptor, thus triggering a series of reactions that eventually effects the Crp-cAMP complex, a dual regulator complex that regulates the expression of the DNA sequence. cAMP binds to Crp, thus changing the form of Crp, and this newly conformed Crp attaches to the promoter, increasing the affinity of the DNA sequence to RNA Polymerase. Crp is also known as CAP, which stands for catabolite activator protein and is present in E. Coli under normal conditions. After RNA Polymerase binds to the promoter and then initiates transcription. By placing GFP after the aldB coding sequence, we will ensure that GFP is also expressed and thus, in the presence of ethanol, the bacteria will glow.

XylR Gene (Detects Xylene)

The genetically-related expression of the XylR gene consists of many steps and parts, including promoters, regulator complexes, and proteins. The first part is the Pr promoter, which promotes the coding DNA sequence for the XylR gene itself. Following that, there is the ribosomal binding site portion of the sequence, which ensures that translation is initiated and that it occurs in the right place. Directly after this RBS is the exon itself, which naturally expresses the XylR protein, important to the E. Coli cell. Because this protein triggers a secondary response in the bacterium which is vital to Xylene detection, a double termination sequence is need to ensure that anything downstream of the XylR coding region is not expressed, potentially disrupting the reactions involved in the detection system. These stop codons, although short, are effective in terminating the expression of the sequence. The XylR protein that was created, given the that xylene is present in the bacterium, reacts with the VOC and becomes conformed. This newly conformed version of the protein can bind to the Pu promoter, which is part of an entirely different sequence. After a second ribosomal binding site (strong) is used after the promoter and YFP is placed directly after this RBS.1, the bacteria will grow green in the presence of xylene.