Nature Structural Biology pp 141 - 146

Tuberculosis is not just a disease of the past—it still kills an estimated three million people per year, and its prevalence is increasing at an alarming rate around the world. Tuberculosis is caused by bacteria with the scientific name Mycobacterium tuberculosis, which take up residence in the lungs of an infected person and wreak havoc. Now, scientists have determined the atomic resolution structure of M. tuberculosis antigen 85c, a protein that could prove to be an effective tool in the effort to combat this disease.

Although a vaccine called bacille Calmette-Gu�rin (BCG) is widely used in developing countries to immunize infants against tuberculosis, it is only partially effective. Nevertheless, infection rates declined until the mid 1980s, thanks to the use of anti-tubercular antibiotics. However, a number of factors, including the HIV/AIDS epidemic and poor compliance of patients with antibiotic treatment regimens, have recently contributed to a resurgence of this disease around the world. Drug resistant M. tuberculosis strains have begun to emerge. Thus, a clear need for additional drugs and vaccines exists to prevent the spread of tuberculosis.

In the February issue of Nature Structural Biology, James Sacchettini, of Texas A&M University in the USA, and his coworkers present the structure of the M. tuberculosis antigen 85c protein. This protein plays a major enzymatic role in synthesizing the cell wall—a resilient coating that surrounds M. tuberculosis, protects it from environmental assaults, and also prevents the entry of certain antibiotics. In addition, Antigen 85c is recognized by the immune systems of individuals infected with tuberculosis, which produce antibodies directed against this protein.

Sacchettini and colleagues determined the structure of antigen 85c using X-ray crystallography, a method that reveals details at the level of individual atoms. The structure helps to explain how antigen 85c performs its chemical reaction and also reveals that the immune-sensitive regions are highly exposed on the surface of the protein.

Antigen 85c is potentially a good target for use in the development of novel drugs and vaccines. If this protein's chemical synthesis activity could be turned off by the binding of specific drugs, then cell wall construction would be stalled, and the bacteria would become more vulnerable. In addition, if the immune-sensitive parts of the protein could be produced in isolation and safely administered to people to elicit antibody production, researchers might have the makings of an effective vaccine. The new structural information should allow scientists to make better guesses about which drugs might interact most effectively with antigen 85c and should also help in pinpointing which surface regions to use in vaccine development. Peter Tonge discusses these results in an accompanying News and Views report, and the Editorial also focuses on tuberculosis.

Nature Structural Biology pp 134 - 140

In 1978 when the Nobel Prize in Physiology or Medicine was awarded to Werner Arber, Dan Nathans and Hamilton Smith for their discovery of "restriction enzymes and their application to problems of molecular genetics", it was clear that these enzymes would become invaluable tools to manipulate, map and study the DNA that makes up every organism's genetic code. This prediction has been borne out in large part because each restriction enzyme cleaves a unique DNA sequence or target site. In fact, a single variation in the DNA sequence results in over a million-fold reduction in activity. How this remarkable specificity is achieved has until now remained elusive.

By comparing the structure of the BglII enzyme bound to its target sequence (AGATCT) with that of the DNA-bound structure of BamHI, an enzyme that cleaves at a very similar target site (GGATCC), Aneel Aggarwal, of the Mount Sinai School of Medicine in New York, and his coworkers have made some progress in understanding restriction enzyme specificity.

These researchers find that both enzymes share a similar core structure but that BglII has an added part that encircles the DNA to provide extra specificity. Surprisingly, the way in which these two restriction enzymes interact with the common central four bases (GATC) differs in the two structures despite the conservation of the amino acids that are responsible for the contacts. Moreover, the DNA bends differently in the two structures leading to completely different DNA-protein interactions.

These findings begin to explain the extraordinary specificity of restriction enzymes and why attempts to switch their specificity have failed. Given that there are only 10 restriction endonuclease structures known to date and over 3,000 different restriction enzymes, we still have some way to go to understand the different strategies for site-specific recognition by this large family of enzymes. Eric Galburt and Barry Stoddard, of the Fred Hutchinson Cancer Research Center, discuss these studies in an accompanying News and Views report.