Tubulin from bovine brain was polymerized in vitro using a variety of assembly conditions. Many of the formed microtubules were shown to contain 14 wall protofilaments. The number of microtubules containing 14 protofilaments increased with consecutive repetitions of cold-dissociation followed by reassembly in vitro.

it is possible that pronase enhances the ability of the pituitary to respond to secretory stimuli.The mechanism of this enhancement is not clear, and the biochemistry of the events which intervene between the addition of prostaglandin E2 or increase in the K + concentration and the release of growth hormone is equally not known. Prostaglandin E2 may act by increasing the concentration of cyclic AMP in the pituitary (2), and high K § leads to depolarization (6) and increased Ca 2+ incorporation (11) without changing the cyclic AMP concentration (10). Since pronase potentiates the effect of both stimuli it is probable that it affects a fundamental property of the secretory system, for example, membrane Ca 2+ permeability.Alternatively, since the release of growth hormone is known to be inhibited by the hypothalamic peptide somatostatin (i), pronase could increase release by destruction of somatostatin present in the tissue at the time it was removed from the animal.We thank Mrs M. Sidler-Ansermet and Miss M. Ravazzola for help.

Mouse fibroblasts (L-929) and Vero (green monkey kidney) cells were infected with the rickettsia Coxiella burnetii, and persistent infections developed and were studied over a 6to 10-month period. Ultrastructural comparisons were made between the two infected cell types, and both were tested cytochemically for the presence of acid phosphatase, a marker enzyme of lysozymes. Rickettsiae were always observed within vacuoles, and some infected L cells showed flattened endoplasmic reticulum as compared with uninfected cells. Rickettsiae in Vero cells were most often seen in vacuoles containing whorls of membranes ("myelin configurations") which were also seen in uninfected cells. Rickettsiae in Vero cells were pleomorphic, with acid phosphatase reaction product in their periplasmic space. This suggests either rickettsial degradation by lysosomal enzymes which penetrated the cell envelope or a penetration after the rickettsiae were dead. Vacuoles of infected Vero cells showed much more reaction product than that in infected L cells, and most rickettsiae in L cells had a normal appearance and showed no reaction product in their periplasmic space.

Ultrastructural studies of single and serial sections of bullfrog olfactory axons showed that smooth endoplasmic reticulum (SER) tubules, which usually appear as single profiles in cross-sections of axons, are continuous over