Translational Components in Prokaryotes: Genetics and Regulation of Ribosomes


The regulation of the biosynthesis of ribosomes, which constitute the catalytic organelles for the translation reaction, is central for the adaptation of bacteria to different growth conditions. The synthesis of the different ribosomal ribonucleic acid (rRNA) and ribosomal protein (RP) components is controlled and coordinated by a complex network of regulatory mechanisms to adjust the translational capacity to the required cell demands. Regulation of rRNA transcription plays a key role in ribosome formation. Quality control steps are operated during processing and assembly to ensure functionally competent particles and to avoid the accumulation of defective products and waste of energy and resources.

Key concepts:

  • Bacterial growth rates depend on the number of ribosomes.

  • Ribosome synthesis is adapted to environmental changes.

  • Biosynthesis of ribosomes involves quality control steps.

  • Ribosome synthesis is determined by the rate of rRNA transcription.

  • Synthesis of many ribosomal proteins is adjusted to the amount of free rRNA by a translational feedback mechanism.

  • Many ribosomal proteins have extra‚Äźribosomal functions in the cell.

  • A limited number of sequence heterogeneities in the redundantly encoded rRNA genes provide the potential for the formation of specialized ribosomes.

  • The stringent control is one of the major mechanisms for the adaptation of ribosomes in response to nutritional changes.

Keywords: ribosome biogenesis; ribosomal proteins; ribosomal RNA; regulation; quality control

Figure 1.

Location and organization of ribosomal components on the E. coli chromosome. (a) Location of ribosomal RNA and ribosomal protein transcription units on the chromosomal map of E. coli. Transcription directions are indicated by arrows; oriC denotes the origin of replication (reproduced from Wagner R (2000) Transcription Regulation in Prokaryotes. By permission of Oxford University Press. (b) rRNA operon gene arrangements. The (UAS) and the tandem promoters (P1, P2) are shown in red and the transcription terminators (T1, T2) are in blue. Structural genes are shown as green boxes. The position of the leader and spacer sequences are marked. Elements in square brackets are not present in all seven rRNA operons.

Figure 2.

Translational feedback regulation for the synthesis of RPs. (a) Mechanism for translational feedback of the S10 operon. L4, 1 of 11 genes of the S10 operon serves as translational repressor. At excess 23S rRNA it binds to the rRNA and enables ribosome formation. Under those conditions the S10 operon is continuously translated. If there is not enough 23S rRNA in the cell L4 accumulates and now binds to a target site within the leader region of the S10 operon. Binding to this site inhibits translation of the complete S10 operon. Hence, translation of the RP genes depends on the availability of rRNAs (reproduced from Wagner R (2000) Transcription Regulation in Prokaryotes. By permission of Oxford University Press. (b) Ribosomal protein operons regulated by translational feedback. The name of each operon is given. P denotes the transcription start site. Individual genes of the operon are shown as green boxes and labelled according to the gene product. The regulatory product is indicated by a blue box. Genes labelled with + are under translational feedback regulation; genes indicated by − are not. In case of the L10 operon the regulator is a complex of L10(L7/L12)4.

Figure 3.

Schematic distribution of sequence heterogeneities among the seven E. coli rRNA operons. Red lines indicate the approximate positions of sequence heterogeneities between the different operons with respect to the rrnB operon, which is taken as reference. Changes occur in all rRNAs with the exception of the 16S rRNA from the rrnE operon, the 5S rRNA from the rrnE operon and the 23S rRNA from the rrnG operon, which are identical with the respective rrnB sequences.

Figure 4.

Transcriptional control elements within a ribosomal RNA transcription unit. The upper line represents a typical rRNA transcription unit, as shown in 1. In the lower part the regulatory region is shown enlarged. The promoter recognition elements (−10, −35) and the transcription start sites (+1) are shown for the P1 and P2 promoter. The UP element, which contacts the RNA polymerase α subunit during transcription initiation, and the discriminator sequence responsible for stringent and growth rate regulation are indicated as grey boxes. Binding sites for the activator protein FIS are shown in green, whereas sites for the repressing transcription factors H‐NS and LRP are given in red or magenta, respectively. The position of the nut‐like leader sequence involved in antitermination and ribosome biogenesis is shown as a grey box. UAS, upstream‐activating sequence.



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Wagner, Rolf(Dec 2009) Translational Components in Prokaryotes: Genetics and Regulation of Ribosomes. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000538.pub2]