The bacterium Escherichia coli possesses ribosomes, which are cellular machines responsible for producing proteins. All living cells contain these structures because the proteins they create are necessary for countless processes, from repairing damage to facilitating metabolic reactions. A ribosome functions by translating genetic instructions into functional proteins, an activity that ensures the cell’s survival and ability to reproduce.
Function of Ribosomes in Bacteria
The primary role of the ribosome in bacteria like E. coli is protein synthesis, a process known as translation. During translation, the ribosome reads the sequence of a messenger RNA (mRNA) molecule, which is a copy of a gene from the bacterium’s DNA. The ribosome moves along the mRNA, reading its code three letters at a time.
For each three-letter code, or codon, a specific amino acid is brought to the ribosome by a transfer RNA (tRNA) molecule. The ribosome then links these amino acids together, forming a long chain called a polypeptide. This chain folds into a specific three-dimensional shape to become a functional protein, allowing the bacterium to build the components it needs to survive and replicate.
Structural Differences in E. coli Ribosomes
While all ribosomes perform protein synthesis, their structure is not identical across all life forms. The ribosome in E. coli, a prokaryote, is structurally distinct from the ribosomes in human cells, which are eukaryotic. These differences are measured using Svedberg units (S), a measurement of particle size and shape. Bacterial ribosomes are designated as 70S ribosomes.
The 70S ribosome is composed of a small 30S subunit and a large 50S subunit. In contrast, human ribosomes are larger, designated as 80S, and are made of a 40S small subunit and a 60S large subunit. The bacterial 30S subunit contains 16S ribosomal RNA (rRNA) and about 21 proteins, while the 50S subunit contains 23S and 5S rRNA alongside approximately 34 proteins.
Targeting Ribosomes with Antibiotics
The structural distinctions between bacterial 70S and human 80S ribosomes are exploited in medicine. This difference allows certain antibiotics to selectively target and inhibit bacterial ribosomes while leaving human ribosomes unaffected, a principle called selective toxicity. This allows the drug to fight an infection without harming the patient’s cells.
Different classes of antibiotics interfere with the bacterial ribosome in specific ways. Tetracyclines, for example, bind to the 30S subunit, blocking tRNA molecules from attaching and thus halting protein production. Other antibiotics like macrolides bind to the 50S subunit, obstructing the tunnel where the new protein exits, which stops synthesis and kills the bacterium.