A simplified translation system coupled to DNA transcription that involves assaying the synthesis of the first dipeptide of a gene product has been described recently Procx NatL Acad. Sci USA 78,[4261][4262][4263][4264]. Using this dipeptide system, we have investigated the expression ofgenes carried on plasmids coding for (8-lactamase, ribosomal protein L12, and the chloroplast large subunit (LS) of ribulosebisphosphate carboxylase (RbuBPCase). Although all three nascent gene products begin with the sequence fMet-Ser, the formation of fMet-Ser can be used to distinguish between the synthesis of .8-lactamase and either L12 or the LS of RbuBPCase by using different serine isoacceptor tRNA species. In ,B-lactamase, the serine codon is AGU, which utilizes the serine isoacceptor species tRNASer; in L12 and the LS of RbuBPCase, the serine codewords are UCU and UCA, respectively, both of which are recognized by the serine isoacceptor species tRNA er. By using either pure tRNA1er or pure tRNA3er, the expression of each gene can be quantitated. In this system, guanosine-5'-diphosphate-3'-diphosphate inhibits the expression ofthe ,j-lactamase and L12 genes but stimulates the synthesis of the LS. In addition, the ratio of fMet-Ser/fMet-Ala (L12/L1O) synthesized was about 1 as compared with the ratio of 4 that has been obtained previously in vivo or in in vitro protein-synthesizing systems, in which the entire gene product was measured. A simplified DNA-directed coupled protein-synthesizing system has been developed to study the regulation ofthe synthesis of ribosomal protein L10 (1). In these initial experiments, plasmid pNF1337 DNA (2) was used as template, and the formation ofthe first dipeptide (fMet-Ala) ofprotein L10 was determined. An important advantage of this DNA-dependent system is that it can be constructed with only five highly purified proteins, i.e., RNA polymerase, initiation factor 1 (IF-1), IF-2, IF-3, and elongation factor Tu (EF-Tu). By using fMet-tRNA and the appropriate aminoacyl-tRNA species for the second amino acid, a specific dipeptide gene product can be selectively synthesized. Because dipeptide formation from a DNA template involves accurate transcription and proper initiation of translation, this system is ideally suited for studies on the regulation of gene expression.The ideal templates are plasmids or purified mRNAs that direct the synthesis of a limited number of products with different second amino acids. Plasmid pBR322 and recombinant plasmids derived from pBR322 contain the ,B-lactamase gene (3), which is efficiently expressed. Thus, the amino-terminal -fragment of ,B-lactamase (fMet-Ser) will be synthesized in the dipeptide system when fMet-tRNA and Ser-tRNA are present. A difficulty arises when a plasmid has another gene that also contains the serine codeword as the second amino acid. This is, in fact, the case with two recombinant plasmids derived from pBR322 that are described in this study. Plasmid pNF1337 contains, in addition to the fl-lactamase gene, the genes for LIO,...