Although viruses are not organisms, they have nevertheless contributed enormously to the scientific understanding of life. This box focuses on a virus that has never made you sick because it does not infect human cells. Rather, it kills Escherichia coli bacteria that live in the intestines of humans and other mammals. Bacteriophage lambda (Greek letter λ) injects its genetic material into its host, which subsequently turns into a virus-making factory. In so doing, phage lambda has revealed many processes fundamental to all life.
Phage lambda’s double-stranded DNA genome of 50,000 base pairs contains all the information it takes to make more phages, and its 60 or so genes must turn on and off in proper sequence for the new viruses to form properly. Biologists learn the functions of viral proteins by studying viral mutants—phages with missing proteins. These studies have revealed that phage lambda’s proteins fall into three general groups:
Phage lambda’s protein coat consists of a nearly spherical head (enclosing the viral DNA) and a tubeshaped tail with proteins that bind to E. coli’s surface. DNA enters the host cell through the tail.
Some of phage lambda’s proteins bind to viral DNA and either promote or prevent transcription of particular genes. Other regulatory proteins stop the virus from entering the lysogenic pathway or prevent the replication of other viruses that may have also infected the cell.
A protein called integrase helps integrate phage lambda’s DNA into the host cell’s chromosome when it enters the lysogenic phase. Another enzyme cuts the viral DNA back out of the chromosome when the lytic phase begins. The balance between these two enzymes determines whether the virus enters the lytic or lysogenic phase.
Why study gene regulation in bacteriophages? Biologists can learn a lot about complex systems by studying the simplest models, since similar mechanisms of gene repression and activation also work in our own cells. When those mechanisms fail, cancer may result.
Phage lambda has taught scientists about not only gene regulation but also recombination and protein folding, both of which are fundamental life processes. In addition, this virus has become something of a laboratory workhorse. Phage lambda is unusual because it can complete its replication cycle even if a large amount of foreign DNA is inserted into its genome. This discovery has made phage lambda an extremely important tool for ferrying recombinant DNA into E. coli.
Other Model Organisms
Mus Musculus (House Mouse); Immune function, Human disease, X chromosome inactivation and stem cells
Drosophila melanogaster (Fruit Fly);Heredity, Human disease, Animal development and circadian rhythms
Arabidopsis thaliana (Mouse-Ear Cress); Control of gene expression, Genome duplication, Disease resistancei, Response to the environment, Hormones, Circadian rhythms and flowering.
Caenorhabditis elegans (Nematode); Animal development, Apoptosis, Muscle function, Drug development, aging and origin of sex.
Neurospora crassa (Red Bread Mold); Crossing over, One-gene/one-enzyme hypothesis, Circadian rhythms and gene regulation.
Dictyostelium discoideum (Cellular Slime Mold); Cell movement, Cytokinesis, Chemotaxis and cell differentiation
Escherichia coli (Bacteria); DNA is genetic material, DNA replication, Gene regulation, Transgenic technology and gene function