Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 235

Type VI secretion systems (T6SS) are very large, with up to 21 proteins encoded within the gene cluster, 12 of which are highly conserved and are thought to play structural roles in the secretion apparatus. At least some of the others unique to each system may encode effectors specifically transported by that system. An intriguing observation has been made that some of the highly conserved proteins structurally resemble components of phage tails (see chapter 7). The highly conserved protein Hcp resembles gp19, the major tail protein of phage T4, and another, VgrG, resembles the syringe of the baseplate of phage T4, consisting of gp27 and gp5. This has led to speculation that T6SS are derived from phage genes and act as inverted phage tails that now eject proteins from the bacterial cell instead of injecting DNA (and sometimes proteins) into the cell.

These systems are required for the secretion of proteins involved in pathogenesis and symbiosis, but they are also found in marine and soil bacteria, where they may have roles in biofilm formation and cell-to-cell communication. There can be little doubt of the importance of these systems, since mutations in their genes can have dramatic phenotypes, but many questions remain about their structure and function.