Ultrastructural and biochemical observations on interphase nuclei isolated from chicken erythrocytes

Adult hen erythrocyte nuclei are isolated from cells or haemolysed in situ by acting on the plasma membrane with rotating knives or with non-ionic detergents. When the isolation medium contains magnesium ions (1 mM), sucrose (0-4 M) and Tris buffer (0.01 M, pH 7-5) called SMTOG (see text), the ultrastructure in thin sections through the condensed chromatin bodies, after staining with either uranyl-lead or phosphotungstic acid (PTA), is similar to that found in the intact cell. Hence it can be concluded that the 2 phases which comprise chromatin, the o- and e-phase, survive nuclear isolation. These are so called because the structural units in chromatin are arranged at the surface of the nucleus into one or more layers and give rise to oddly (o) and evenly (e) numbered bands. The 0-phase is also largely retained after extensive washing in 0-07 M NaC1 as shown by electron microscopy and biochemical measurements; only 6% of the total nuclear protein is removed, a value small compared with the fractional amount of the chromatin protein calculated to lie in the o-phase, about 70%. After extensive washing in saline-EDTA there are structural changes in chromatin, but biochemical data show that the molecules in the o-phase are also largely retained; loss of protein amounts to between 5 and 11%. These data suggest that the o-phase is a structural component of the chromatin bodies. They support the hypothesis that condensed chromatin is formed by folding superunit threads. These units consist of a central thread-like element about 17 nm diameter which stains preferentially with uranyl-lead and forms the e-phase, with an outer cylindrical shell forming the o-phase of total diameter about 28nm. The 5-10% proteins removed by salt washes are located exclusively in a particulate component, quite likely the chromatin. They have been examined by sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis. There are about 10 or more protein species, ranging in molecular weight from 21000 upwards. The groups of large granules previously found in the nuclear sap of intact erythrocytes are shown to be associated with an amorphous or finely fibrillar body.