Scientists say that these bacteria have modified what has long been considered typical cell behavior by using a beta form of an amino acid – as opposed to an alpha form – during the act of making proteins.
Beta versions of amino acids occur in nature under rare and specific circumstances, but have never been observed as part of protein synthesis. Before this finding, in fact, researchers had determined that virtually all proteins were constructed with the alpha forms of amino acids.
This work has shown that when researchers delete any one of three genes from the process that makes use of the beta form of the amino acid, or if they insert the alpha form in the beta version's place, Salmonella cells are no longer able to cause disease. The amino acid in question is lysine, one of 22 genetically encoded amino acids that are strung together in cells to make proteins.
"When these genes were knocked out, the cells became sensitive to antibiotics. And if we put beta lysine into the medium where cells were growing, they became resistant to antibiotics," said Michael Ibba, professor of microbiology at Ohio State University. "So we could see the beta amino acid being taken up and used. The cells really do need the beta amino acid to be resistant to antibiotics, and for other aspects of their virulence."
Under normal circumstances in cells, an enzyme will select amino acids in the cell and place them on a molecule called transfer RNA, or tRNA, which leads to translation of the genetic code into proteins. In Salmonella cells, these steps are similar, but with a few surprising twists, Ibba said. He and colleagues confirmed that the YjeK gene makes beta lysine, and showed that the PoxA gene takes that beta lysine and attaches it to EF-P – a protein that partially mimics the shape and function of tRNA.
The research team first reconstructed this unusual protein synthesis process in test tube experiments, and then followed with studies in cell cultures. Even before they took on studying the mechanism, however, they knew that the effects of these virulence genes were powerful: In earlier animal studies, deleting any one of the three genes and then infecting mice with these altered Salmonella cellshad no effect on the animals. When the genes were left intact and cells were injected into mice, the resulting Salmonella infection killed the animals.
"This tells us the cell is not going to be able to easily replace the beta amino acid," Ibba said. "It is essential for virulence in Salmonella." And that, he said, is why that amino acid might be such an effective drug target, especially as humans don't seem to make beta amino acids at all.
"You have to make an antibiotic look like something natural, only different. If you have something that's already different like a beta amino acid, you've potentially got a much better drug target because it involves chemistry that's comparatively rare in the cell. It's harder for the cell to try to alter its own chemistry to develop resistance," Ibba said.
MEDICA.de; Source: Ohio State University