introduction of an oligonucleotide tgactgactga encoding stop codons in all three frames into the blunt ended clai site at the cdna 3 39 terminus while the bamhi cleavage site at the cdna 5 39 terminus was deleted by filling in using the klenow fragment of e coli dna polymerase i cells transformed with this plasmid psj10 u03b4bam7stop7 and induced with iptg synthesised a mr 27 500 polypeptide but at less than 20 of the level in cells transformed with psj5 a plasmid derived from ptac11 encoding native sj26 smith et al 1987b this difference in expression may be due to the increased gc content and length of the region between the tac ribosome binding site and the atg translation initiation codon in psj10 u03b4bam7stop7 stormo et al 1982 de boer et al 1983b and so the 3 39 terminus of the modified sj26 cdna in psj10 u03b4bam7stop7 containing the multiple cloning sites and termination codons was introduced into psj5 as follows a hindiii ndei fragment of psj5 encoding the tetracycline resistance gene was replaced with a 1 7 kb ecori fragment derived from pmc9 miller et al 1984 containing the laci q allale and a portion of lacz blunt end ligation of purified fragments after treatment with the klenow fragment of e coli dna polymerase i produced a plasmid p4.