A. Structure and Composition
Ribosomes are composed of ribosomal RNA (rRNA) and protein. Prokaryotic cells have three types of rRNA: 16S rRNA, 23S rRNA, and 5S rRNA. Like transfer RNA (tRNA), rRNAs use intrastrand H-bonding between comlementary nucleotide bases to form complex folded structures.
Ribosomes are composed of two subunits with densities of 50S and 30S. (A svedberg unit (S) is a non-metric unit for sedimentation rate and is actually a measure of time defined as 10-13 seconds. In biology, the sedimentation rate, or sedimentation coefficient, refers to the rate at which a molecule or particle travels to the bottom of a test tube under the centrifugal force of an ultra-high speed centrifuge. The S value of a molecule or particle is determined by its mass, density, and shape.) The 30S subunit contains 16S rRNA and 21 proteins; the 50S subunit contains 5S and 23S rRNA and 31 proteins. (See Ribosomal subunit sedimentation rates: 50S and 30S.)
The two subunits combine during protein synthesis to form a complete 70S ribosome about 25nm in diameter. A typical bacterium may have as many as 15,000 ribosomes.
To view electron micrograph showing the nucleoid and ribosomes in a Streptococcus pneumoniae. see Fig. 1.
B. Functions
Ribosomes function as a workbench for protein synthesis, that is, they receive and translate genetic instructions for the formation of specific proteins. During protein synthesis, mRNA attaches to the 30s subunit and amino acid-carrying transfer RNAs (tRNA) attach to the 50s subunit (see Fig. 2). Protein synthesis is discussed in detail in Unit 6.
The chromosome is the genetic material of the bacterium. Genes (def) located along the DNA are transcribed (def) into RNA molecules, primarily messenger RNA (mRNA) (def), transfer RNA (tRNA (def), and ribosomal RNA (rRNA) (def). Messenger RNA is then translated (def) into protein at the ribosomes.
Transcription
Ribonucleic acid (RNA) is synthesized by complementary base pairing (def) of ribonucleotides (def) with deoxyribonucleotides (def) to match a portion of one strand of DNA called a gene (def). Although genes are present on both strands of DNA, only one strand is transcribed for any given gene. Following transcription of genes into mRNA (def), 30S and 50S ribosomal subunits attach to the mRNA and tRNA inserts the correct amino acids which are subsequently joined to form a polypeptide or a protein through a process called translation.
Translation
During translation, specific tRNA molecules (def) pick up specific amino acids, transfer those amino acids to the ribosomes, and insert them in their proper place according to the mRNA (def) "message." This is done by the anticodon (def) portion of the tRNA molecules complementary base pairing (def) with the codons (def) along the mRNA.
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by Gary E. Kaiser, Ph.D. Last updated: August, 2019 |
In bacteria, transcription and translation (def) are coupled. RNA polymerase (def) binds to the 30S ribosomal subunit of prokaryotic ribosomes to form a transcription and translation machine called an expressome. As the DNA is being unwound and transcribed into complementary mRNA by RNA polymerase, the mRNA is being fed into the translational center of the ribosome where it is being translated into a polypeptide. Translation is described in the next lesson.
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C. Antimicrobial Agents that Alter Prokaryotic Ribosomal Subunits and Block Translation in Bacteria
Many antibiotics alter bacterial ribosomes (def), interfering with translation (def) and thereby causing faulty protein synthesis. The portion of the ribosome to which the antibiotic binds determines how translation is effected. For example:
- The tetracyclines (tetracycline, doxycycline, demeclocycline, minocycline, etc.) bind reversibly to the 30S subunit, distorting it in such a way that the anticodons of charged tRNAs (def) cannot align properly with the codons of the mRNA (def).