14

key concepts

14.1

Genes Code for Proteins

14.2

Information Flows from Genes to Proteins

14.3

DNA Is Transcribed to Produce RNA

14.4

Eukaryotic Pre-mRNA Transcripts Are Processed prior to Translation

14.5

The Information in mRNA Is Translated into Proteins

14.6

Polypeptides Can Be Modified and Transported during or after Translation

Employing the Genetic Code to Combat Superbugs

Staphylococcus aureus is among billions of bacteria that inhabit the human skin and nose, mostly with no ill effect. But sometimes, especially when the immune system has been weakened by age or disease, S. aureus can cause major skin infections and may enter the body through the nose or a wound site, causing serious and sometimes fatal infections of organs such as the heart and lungs.

Until recently, most S. aureus infections were successfully treated with penicillin and related drugs, including methicillin. These antibiotics bind and inactivate several related enzymes involved in the assembly of bacterial cell walls. New bacterial cells produced by cell division of antibiotic-treated bacteria do not survive.

Unfortunately, some S. aureus strains have acquired mutant versions of a penicillin-binding protein that are resistant to antibiotics—the mutant enzyme can catalyze the assembly of cell walls even in the presence of antibiotics. The mutant mecA gene that encodes this mutant protein can be passed from one bacterium to another by bacterial conjugation (see Key Concept 12.6).

What makes the mutant version of mecA different, and how does this difference lead to antibiotic resistance? Compared to the wild-type allele, the mutant mecA gene has a small change in its nucleotide sequence. This results in changes in the amino acid sequence of the expressed protein. This alteration in the protein’s secondary structure affects its tertiary structure: the protein folds into a shape that doesn’t bind the antibiotics.

The fundamental lesson conveyed by this example is that a gene is expressed as a protein, and more specifically, that the sequence of nucleotides in DNA is expressed as a sequence of amino acids in a protein. Understanding how the nucleotide sequence of a gene produces a specific protein came with the discovery of the genetic code, a landmark in biological research.

Mutant bacterial strains that are stubbornly resistant to antibiotics are known as “superbugs.” One greatly feared superbug is methicillin-resistant S. aureus, or MRSA. About 1 person in 50 carries MRSA, so it represents a real threat to public health. MRSA incidence is declining, thanks to careful monitoring and treatment in hospitals and nursing homes, where people are especially vulnerable. The infection is treated with a new class of antibiotics that target bacterial protein synthesis.

How does knowledge of the genetic code help us understand the actions of some antibiotics?