THE BACTERIAL NUCLEOID REVISITED

This review compares the results of different methods of investigating the morphology of nucleoids of bacteria grown under conditions favori ng short generation times. We consider the evidence from fixed and sta ined specimens, from phase-contrast and fluorescence microscopy of gro wing bacteria, and from electron microscopy of whole as well as thinly sectioned ones. It is concluded that the nucleoid of growing cells is in a dynamic stare: part of the chromatin is ''pulled out'' of the bu lk of the nucleoid in order to be transcribed. This activity is perfor med by excrescences which extend far into the cytoplasm so as to reach the maximum of available ribosomes. Different means of fixation provi de markedly different views of the texture of the DNA-containing plasm of the bulk of the nucleoid. Conventional chemical fixatives stabiliz e the cytoplasm of bacteria but not their protein-low chromatin. Urany l acetate does cross-link the latter well but only if the cytoplasm ha s first been fixed conventionally In the interval between the two fixa tions, the DNA arranges itself in liquid-crystalline form, supposedly because of loss of supercoiling. In stark contrast, cryofixation prese rves bacterial chromatin in a finely granular form, believed to reflec t its native strongly negatively supercoiled state. In dinoflagellates the DNA of their permanently visible chromosomes (also low in histone -like protein) is natively present as a liquid crystal. The arrangemen t of chromatin in Epulocystis fishelsoni, one of the largest known pro karyotes, is briefly described.