Lac repressor mRNA transcription terminates in vivo in the lac control region.

We have isolated the in vivo messenger RNA encoding the lactose repressor protein of Escherichia coli by hybridization to lacI gene DNA, and have verified its identity by characterizing specific fragments derived from its 5' and 3' ends. The 3' end points of the RNA, as shown by oligonucleotide analysis and S1 nuclease-mapping data, are clustered in the lac control region, within 70 nucleotides beyond the end of the repressor protein coding sequence. DNA fragments from this region, when inserted between the E. coli gal operon promoter and galactokinase (galK) gene, effect an 80% reduction in downstream galK expression. These results are indicative that sequences at the gene end function in vivo to terminate lacI gene transcription.