Our examination of the regulation of mRNA synthesis begins by considering some characteristics of mRNA, the sequence of which is determined by a specific template DNA sequence within the bacterial chromosome. Because transcription proceeds in a 5'→3'direction and transcription and translation occur in the same cellular compartment, the bacterial protein synthetic machinery can start to read the 5'-end of mRNA before the 3'-end is formed. Therefore, a bacterial cell does not have a chance to alter a nascent mRNA molecule before the protein synthetic machinery begins to translate it. The situation is different in eukaryotes. Because transcription occurs in the cell nucleus and translation occurs in the cytoplasm, the eukaryotic cell can convert the primary transcript to mature mRNA in the cell nucleus before the mRNA is required to direct protein synthesis in the cytoplasm.
    Protein synthetic machinery leads the mRNA nucleotide sequence in groups of three bases or codons. Each codon specifies an amino acid or a termination signal. The protein synthetic machinery begins polypeptide synthesis at a start codon located toward the5'_end of the mRNA and continues synthesis in a 5'→3'direction until it encounters a termination codon. The segment of mRNA that codes for a polypeptide chain is called an open reading frame (ORF) because the protein synthetic machinery begins reading the segment at a specific start codon and stops reading it at a specific start codon and stops reads it at a specific termination codon. A DNA segment corresponding to an open reading frame plus the translational start and stop signals for protein synthesis is called a cistron and an mRNA encoding a single polypeptide is called monocistronic mRNA. Although the terms cistron and gene are sometimes used interchangeably to describe bacterial DNA segments that specify polypeptides, the term gene has a broader meaning because it also includes the promoter region and applies to DNA segments that code for RNA molecules such as tRNA and rRNA that are not translated.
    Bacterial mRNA molecules often contain two or more cistrons. In fact, bacterial polycistronic mRNA molecules are actually more common than bacterial monocistronic mRNA molecules. Each cistron within a polycistronic mRNA specifies a specific polypeptide chain. Furthermore, cistrons contained in polycistronic mRNA often specify proteins for a single metabolic pathway. For example, one E. coli mRNA has three cistrons, each coding for a different protein required for lactose metabolism and another contains ten cistrons, each coding for a different enzyme required for histidine synthesis.