Bacteria have evolved expression units called operons which unite functionally related genes under the control of a single promoter, thus enabling coordinated, simultaneous and rapid expression of metabolically coordinated genes in response to specific environmental signals or physiological constraints (the classic example is the lactose operon; Dickson et al., 1975) . Individual genes in a bacterial operon are preceded by characteristic sequences, socalled ribosomal binding sites (RBS), for translation-initiation at appropriate points within a single mRNA molecule. In contrast to bacterial multigene transcripts, most eukaryotic mRNAs are monocistronic, and optimal translation of the encoded gene relies on a post-transcriptional 5 modification (capping) for ribosome binding and subsequent AUG-scanning (Shatkin, 1985; Kozak, 1989) . However, other capindependent modes of translation-initiation such as leaky scanning, termination-reinitiation, and internal initiation are used in rare cases (Jackson et al., 1995;  Table 1; Figure 1 ).