GIGANTISM IN A BACTERIUM, EPULOPISCIUM-FISHELSONI, CORRELATES WITH COMPLEX PATTERNS IN ARRANGEMENT, QUANTITY, AND SEGREGATION OF DNA

Epulopiscium fishelsoni, gut symbiont of the brown surgeonfish (Acanth urus nigrofuscus) in the Red Sea, attains a larger size than any other eubacterium, varies 10- to 20-fold in length (and >2,000-fold in volu me), and undergoes a complex daily life cycle. In early morning, nucle oids contain highly condensed DNA in elongate, chromosome-like structu res which are physically separated from the general cytoplasm. Cell di vision involves production of two (rarely three) nucleoids within a ce ll, deposition of cell walls around expanded nucleoids, and emergence of daughter cells From the parent cell. Fluorescence measurements of D NA, RNA, and other cell components indicate the following. DNA quantit y is proportional to cell volume over cell lengths of similar to 30 mu m to >500 mu m. For cells of a given size, nucleoids of cells with tw o nucleoids (binucleoid) contain approximately equal amounts of DNA. A nd each nucleoid of a binucleoid cell contains one-half the DNA of the single nucleoid in a uninucleoid cell of the same size. The life cycl e involves approximately equal subdivision of DNA among daughter cells , formation of apical caps of condensed DNA from previously decondense d and diffusely distributed DNA, and ''pinching'' of DNA near the midd le of the cell in the absence of new wall formation. Mechanisms underl ying these patterns remain unclear, but formation of daughter nucleoid s and cells occurs both during diurnal periods of host feeding and bac terial cell growth and during nocturnal periods of host inactivity whe n mean bacterial cell size declines.