I have cloned a gene for a 10 kDa bacterial protein in pET28a (to be expressed as a C terminal His tagged protein) and transformed into DH5a. I have confirmed the clone by restriction digestion and have sent for sequencing as well. Following fresh transformation into BL21 (DE3) cells and induction of cultures with IPTG (0.5 mM and 1mM) for 3-4 h at 37 degree as well as 24 degree celsius), I failed to see any over expressed protein on SDS-PAGE in pellet or supernatant fraction. Western blot of the same sample probed with anti-His antibody however showed a good band around 10 kDa; specially in the supernatant fraction; however the signal was similar in induced and uninduced control (absent in pET28a empty vector control). Can anyone provide an explanation and also suggest how to move forward with the purification.

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