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Some structural and functional properties of ribosomes from polysomes and from helix aggregates of Entamoeba invadens have been compared by sucrose gradient analysis and assays of in vitro protein synthesis. Actively growing trophozoites, lacking helices, presented normal polysome profiles in sucrose gradients. The single large ribosomal helix aggregate (chromoatoid body) of cysts diappeared as the cells were disrupted. Gradient profiles of cyst extracts contained predominantly large and small ribosome subunit peaks and no evidence of remaining helix fragments of mRNA-bound polysomes. Sequential profiles of trophozoites incubated with NaF or cycloheximide (which both stimulate ribosome aggregation, but at different rates) showed that polysome breakdown occurred before aggregates appeared and, again, that helices broke down to subunits in vitro. Radioactive ribosomes synthesized during vegetative growth were collected into helices during encystation. Subunits of these ribosomes cosedimented with comparable particles isolated from trophozoites. Ribosomes from both trophozoites and cysts were active in cell-free protein synthesis, although activity in cyst extracts required the addition of trophozoite-soluble fraction. It was concluded that ribosomes from polysomes and helices in E. invadens were probably identical and that the ability to form helices was an intrinsic property of mature mRNA-free ribosomes of this organism.

Within recent years the recognition of interstitial plasma-cell pneumonia has increased in America. The problem of its etiology is still unfolding, for now it is recognized here in the United States and in Europe that the majority of instances of interstitial plasma-cell pneumonia are really Pneumocystis carinii infections. This latter condition, like interstitial plasma-cell pneumonia, has been reported often from many European countries but has only recently been observed in the United States.1 Perhaps this circumstance may be due to the fact that it was not looked for in the material in the United States.Pneumocystis carinii is an organism whose taxonomic classification remains controversial. By some it has been stated to be a protozoan, and by others it is called a fungus.2 Chagas first discovered the organism but erroneously believed it was a stage of development of Trypanosoma cruzi.3 After observations by several investigators5-13 it