In vivo overexpression and purification of Escherichia coli tRNA(ser)

DNA fragments corresponding to the sequences of Escherichia coli tRNA(2ser) and amber suppressor tRNA(ser), were synthesized from overlapping oligonucleotides. These were interposed between a strong promotor and a synthetic transcriptional terminator to ensure the production of a transcript of the correct size. The genes of promotor, fragment and terminator were cloned into a conditional runaway replication plasmid. At temperatures below 37 degrees C this vector has a low copy number but, following a temperature shift to 42 degrees C, the copy number is no longer regulated. Using these constructs an overexpression of tRNA(ser) of about 20 times the level of the wild-type pool could be obtained (corresponding e.g. to 200 times the expression tRNA(2ser)). From these systems 10 mg quantities of tRNA(ser)s could be isolated with a serine acceptance of 1,100 pmol/A280 unit.