plasmid vectors with a semisynthetic betagalactosidase gene of e coli a partial synthesis of a structural gene for betagalactosidase and construction of a series of plz plasmids for quantitative study of e coli promoters are reported the gene was assembled of two short synthetic dnas and of a 3000 bp long ecori fragment comprising the lacz sequence 163013 isolated from plasmid p1981 of b gronenborn among the plasmids constructed plz4 is a promoterprobe vector that contains the semisynthetic gene fused with a synthetic shinedalgarno sequence and preceded by unique ecori and kpni cleavage sites on cloning a promoter into these sites its signal strength in vivo could easily be measured by assaying betagalactosidase activity the use of plz4 vector was demonstrated by quantifying the effect of t7 early promoters a1 and a2 the latter being found 45 times more active under the conditions employed