Materials and Methods: 50ul of chemically competent E.Coli (Bl21D3) were used in the experiment. The cells were incubated with pRSET vector, presented by figure 2, containing both GFP insert and ampicillin resistance gene. After thawing a sample on ice to ensure a plasmid intake, transduced BL21D3 mixed with Lysogeny broth (LB) were incubated at 370 for 60 minutes. Following the incubation 20 and 200 ul of sample was introduced on 2 halves of ampicillin enriched Agar plate, figure 3A. The plates were incubated overnight at 170. Following the incubation, a colony was selected and ejected into 10 ml of LB broth with 100 ug/ml ampicillin to create a starter culture. The culture was incubated overnight on a shaker, to prevent cells form settling. To create a lager culture, 1ml of starter culture and 100 ml of 100 ug/ml ampicillin were introduced to 100ml of 2XYT and incubated in 37 0C shaker for 6 hours, to reach the OD600. 50 ul of “pre induced” sample were taken from the culture and injected into an Eppendorf tube. The remaining culture was induced by adding 50ul of IPTG to force the production of rGFP. After the induction the culture was incubated overnight in 370C shaker. Sample of 50 ul of the post induction sample was injected into an Eppendorf. Cells expressing rGFP were harvested by centrifugation and the pellet was weighted and re-suspended in adequate amount of BugBuster TM (5ml of BugBuster per each gram of pellet paste). Following the re-suspension the pellets were centrifuged again at 13krpm for 20 minutes. After the centrifugation a sample of supernatant was conserved. The pellets were re-suspended. A Ni2+ chromatographic column was equilibrated using 10ml of wash buffer (phosphate buffer, pH 8 20 mM imidazole, 300 mM NaCl). A sample of supernatant and flow through were collected. The column was washed with 4 ml of elution buffer (phosphate buffer, pH8, 250 mM imidazole, 300mM NaCl) and elute was collected. SDS-PAGE was runned at 100V for 2 hours. The wells were loaded with 10ul of sample and the Laemil buffer. The gel was stained overnight and examined on a following day.
Results: The goal of the experiment was not fully met. Despite performing a successful plasmid transformation, confirmed by the cellular growth on the antibiotic containing media, the final SDS-PAGE measurement failed to show a decisive evidence of the GFP presence. In this experiment the particular E.Coli strain used, the pRSET vector is repressing the promoter region. The promoter is essential for the polymerase (T7) to work. The binding of T7 polymerase to the promoters of the vector, triggers the polyhistidine tags that express the GFP fluorescence. This enables a low production of rGFP however it is not enough to see a clear purification band on the SDS-PAGE, hence the IPTG induction takes place that stops the repressor from working. As a result, an increased production of GFP should be observed.
The first time an undesirable outcome of the experiment was noticed was after the 6 hour incubation period at 370 C before the bacterial inoculation. The culture should have reached the chosen optical density, OD600, with the liquid being cloudy. At this optical density, the bacteria should still be healthy, in their growing phase, large in numbers. The observed solution was yellowish brown as expected but completely clear which indicated that the chosen optical density was not reached. A number of colleagues had the same issue with their cultures however the bacteria had to be induced regardless of the desiarable OD not being reached.