Myogenin Controls Muscle Metabolism

Streptomyces cell death, which occurs as the bacteria develop fruiting bodies and form spores, has been dismissed as nonspecific autolysis caused by cell wall degradation. Now Miguélez et al. (page 515) show that the cell death is complex: it occurs at two different times in two different locations and as a series of defined steps that leave the cell wall intact. Streptomyces grow first along and through their growth medium, as multinucleoid hyphae. This substrate mycelium subsequently forms stalks that rise above the growth medium. Parts of the aerial hyphae septate to form spores. Cell death occurs in two major waves. As aerial hyphae are growing, nucleoids at the level of the growth medium begin degenerating. Sporulation then starts, by which time the substrate mycelium is dead and a second death process is starting in the aerial hyphae, from the bottom up. Two days later all cells are either dead or spores. The first sign of degeneration is a loosening of the fibrillar DNA structure. The nucleoid and then cytoplasm become less electron-dense, ending with a nucleoid that is electron-translucent and has fibrillar structure. Only at the end of this process does the plasma membrane retract from the cell wall and form vesicles. The cell wall is left intact, probably as both a mechanical support and nutrient conductor for the aerial hyphae. A genetic dissection of the death process would seem to be the next logical step. This may uncover a link to programmed cell death in other organisms, as the Streptomyces genome contains distant relatives of some mammalian apoptosis genes (Aravind, L., V.M. Dixit, and E.V. Koonin. 1999. Trends Biochem. Sci. 24:47–53).