Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 159

Transcription of the genes for all RNAs in the cell follows the same basic process. However, rRNAs and tRNAs play special roles in protein synthesis, so their fates after transcription differ from that of mRNAs.

Figure 2.14 Transcription termination at a factor-independent termination site. (A) DNA sequence of a typical terminator site. (B) Sequence and structure of the RNA hairpin that forms in the nascent RNA as it emerges from RNA polymerase, which (in combination with the weak RNA-DNA hybrid) causes RNA polymerase to dissociate from the template DNA and release the RNA product.

Figure 2.15 Model for factor-dependent transcription termination at a ρ-sensitive pause site. The ρ factor attaches to the RNA at a rut site if the RNA is not being translated (for example, if the ribosome has stopped at a termination codon) and forms a hexameric ring around the RNA. It then moves along the RNA with the cleavage of ATP until it catches up with RNA polymerase paused at a ρ-sensitive pause site. The helicase activity of the ρ factor then dissociates the RNA-DNA hybrid in the transcription bubble, causing the RNA polymerase and the RNA to be released.

The ribosomes are some of the largest structures in bacterial cells and are composed of both proteins and RNA. Bacterial ribosomes contain three types of rRNA: 16S, 23S, and 5S. The S value (from Svedberg, the name of the person who pioneered this way of measuring the sizes of molecules) is a measure of how fast a molecule sediments in an ultracentrifuge. In general, the higher the S value, the larger the RNA. The designation has persisted, even though this method of measuring molecular size is rarely used.

The rRNAs are among the most highly evolutionarily conserved of all the cellular constituents, as indeed are many of the components of the translational machinery. For this reason, they have formed the basis for molecular phylogeny (Box 2.2). Comparisons of the sequences of rRNAs and other constituents of the translation apparatus from different species permit estimates to be made of how long ago these constituents separated evolutionarily.

In addition to their structural role in the ribosome, the rRNAs play a direct role in translation. The 23S rRNA is the peptidyltransferase enzyme, which joins amino acids into protein on the ribosome. The 23S rRNA therefore acts as a ribozyme, an RNA enzyme (see below). The 16S RNA lacks enzymatic activity but plays crucial roles in initiation and termination of translation, as well as in decoding of the sequence of the mRNA.

The rRNAs and tRNAs make up the bulk of the RNA in cells because of their central role in protein synthesis. In a rapidly growing bacterial cell, much of the total RNA synthesis is devoted to making these RNAs. Also, the rRNAs and tRNAs are far more resistant to degradation than mRNA. With this combination of a high synthesis rate and high stability, the rRNAs and tRNAs together can amount to more than 95% of the total RNA in a rapidly growing bacterial cell.

Not only do the rRNAs physically associate in the ribosome, but they also are synthesized together as long precursor RNAs containing all three forms of rRNA separated by so-called spacer regions. The precursors often contain one or more tRNAs, as well (Figure 2.16), while other tRNAs are encoded in operons that do not include rRNA sequences. The individual rRNAs and tRNAs are released from the precursor RNAs by ribonucleases (RNases). Some of these RNases participate in both rRNA and tRNA processing and RNA degradation, while others are dedicated to a single function; for example, RNase P generates the precise 5′ end of tRNAs. At some point during the processing, individual nucleotides within the rRNA and tRNA precursor molecules are also modified to make the mature rRNAs and tRNAs.