Ultrastructural studies of the nucleoids of the pleomorphic forms of Chlamydia psittaci 6BC: a comparison with bacteria

Abstract

The nucleoids of the various pleomorphic forms of Chlamydia psittaci have been examined by direct observation of infected cells and by observations on isolated particles. The fixation and staining methods used were the same as those routinely used for the examination of bacteria to facilitate the comparison of chlamydial fine structure with that of bacteria. The nucleoids of reticulate bodies were composed of fine fibrils which extended throughout these particles. The nucleoids of intermediate bodies are characterized by an electron-dense mass with which the fibrous elements are associated in a structurally coherent manner. As condensation of the intermediate bodies proceeds, the electron-dense mass becomes eccentrically located and the fibers form a distinct radiating structure. Large elementary bodies have a few fibers associated with their condensed electron-dense nucleoids but the more condensed mature elementary bodies have a very discrete and homogeneous electron-dense nucleoid which is separated from the cytoplasmic elements of these particles by a very distinct electron-transparent space. These highly condensed elementary body nucleoids are usually ovoid, but may be elongated or irregular, and a small number of these structures react very strongly with ruthenium red. While the nucleoid structure of reticulate bodies resembles that of the bacterial cell, both the condensation process and the nucleoid morphologies which result from it in intermediate and elementary bodies have no parallels among the bacteria. Thus we conclude that major differences in nucleoid organization exist between the chlamydia and the bacteria.

Global Stage-Specific Gene Regulation during the Developmental Cycle of Chlamydia trachomatis

“…This class of genes is typified by omcA and omcB; expressed only late in the developmental cycle, they encode two cysteine-rich membrane proteins found only in EB (13). In addition to outer membrane changes, EB are characterized by marked DNA condensation late in development (6). A unique DNA topoisomerase fused to an SWIB domain (CT643), a DNA gyrase A paralogue (CT660), and histone-like protein 2 (hctB) are also induced at this stage and are implicated in the mechanism of the terminal stages of condensed nucleoid formation.…”

A developmentally regulated chlamydial gene with apparent homology to eukaryotic histone H1.

“…In addition to significant differences in size and membrane permeability between EBs and RBs, chromatin organization varies markedly between the two forms. Notably, the DNA genome of RBs resembles that of other bacteria, with diffuse fine fibrils extending throughout the cell, whereas mature EBs have a discrete, condensed electrondense nucleoid that appears to be unique among prokaryotes (2).…”

Purification and N-terminal amino acid sequences of Chlamydia trachomatis histone analogs

“…Within about 8 h the EBs differentiate to a larger, pleomorphic, metabolically active but noninfectious cell type known as reticulate bodies (13). During this initial differentiation, the disulfide-linked outer membrane complex is reductively cleaved, allowing increased permeability and porin function (1,9,10,16); the condensed chromatin becomes disperse (3), and the chlamydiae increase in size to a diameter of about 1 gm (3). The reticulate bodies multiply by binary fission until about 24 h postinfection, at which time they begin to undergo a secondary differentiation of reorganization and condensation to EBs prior to release for reinitiation of another cycle of infection.…”