REGULATION OF TRANSLATION OF THE DISTAL L ACZ GENE IN POLYCISTRONIC MESSENGER-RNA BY THE RIBOSOME STREAM FROM A PROXIMAL GENE

Four series of plasmids (pNSI, pNSII, pNLI, and pNLII) with artificial polycistrons containing the lacZ test gene were constructed. These pl asmids coded for polycistronic mRNAs with two different types of cistr on (orfZ and lacZ) coupling: in pNSI and pNLI, the orfZ termination co don and the lacZ initiation codon overlapped (type I); in pNSII and pN LII, the orfZ termination codon was located upstream of the lacZ SD se quence. The length of the orfZ cistron was 60 bp in pNSI and pNSII or 300 bp in pNLI and pNLII. Plasmids with the same type of cistron coupl ing contained the same lacZ translation initiation region, whereas the structure of the orfZ translation initiation region varied, thereby p roviding varying efficiency of the orfZ gene translation. The effect o f these variations on the efficiency of the lacZ gene translation was evaluated by direct measurement of the beta-galactosidase activity in Escherichia coli cells transformed with the corresponding plasmids. We found that the level of translation of the distal lacZ gene depended on the ribosome stream from the proximal gene and was maximal at the o ptimal ribosome stream level, which, in turn, depended on the type of cistron coupling.