The complete nucleotide sequence of an alanine transfer RNA, isolated from yeast, has been determined. This is the first nucleic acid for which the structure is known.

Purified growth inhibitor from BSC-1 cells and type beta transforming growth factor from human platelets are shown to have nearly identical biological activity and to compete for binding to the same cell membrane receptor. These findings suggest that the growth inhibitor and the type beta transforming growth factor are similar molecules. The data also show that the same purified polypeptide can either stimulate or inhibit cell proliferation depending on the experimental conditions.

The 3T3 cell, an established line of mouse fibroblast cell, has been considered to be extremely sensitive to "contact inhibition" of cell division. Under the usual culture conditions, 3T3 cells grow rapidly in sparse culture, but cell division stops after the cells become confluent, at approximately 106 cells per 6-cm dish. The cell monolayer has a typical "cobblestone" appearance. Todaro, Lazar, and Green1 have described studies of the effect of serum on cell division in these "contact-inhibited" cells. The addition of serum to an inhibited culture leads to a rise in RNA synthesis, followed in a few hours by protein synthesis, and eventually by some DNA synthesis. Todaro, Lazar, and Green1 have inferred that a factor in serum overcomes "contact inhibition" of cell division. In the experiments described below, we find that the characteristic "contact-inhibited" cell density observed for 3T3 cells is a fortuitous result of growing the cells in a medium that contains 10 per cent calf serum. The final cell density, after cell division stops, is directly proportional to the amount of serum added to the medium.2 The experiments suggest that serum contributes a factor or factors required by 3T3 cells for cell division. Viral-transformed 3T3 cells have a greatly reduced requirement for the serum factor(s).Whether serum factors offer an explanation of "contact inhibition" of cell division in other instances remains to be determined. It is pertinent that Temin3 has found that an insulin-like factor in serum is required for cell divison by cultured chick cells. Transformation of chick cells by Rous sarcoma virus lowers the requirement for this serum factor. Materials and Methods.-The 3T3 cell line was obtained from Dr. Marguerite Vogt. The cell line had been obtained originally from Dr. Howard Green and had been cloned recently by Dr. Vogt to maintain the typical "cobblestone" appearance. During prolonged culture, 3T3 cells gradually lose their high requirement for serum and grow to higher cell densities. Therefore, the cell line was maintained at -90'C, and cells used in the experiments were not cultured over 8 weeks. The cells were grown in enriched Eagle's medium, as used in Dulbecco's laboratory.4 To count the cells, the medium was removed from the dishes, the cell layer was trypsinized with half the concentration of trypsin used during transfer of the cells, and the cells were counted in the trypsin solution by means of a hemacytometer. Counts were on duplicates. Experiments were replicated at least three times. The standard error observed for replicate counts was approximately 10%.Assay for growth factor: Approximately 105 3T3 cells were plated per 6-cm plastic dish in 5 ml of medium with 6% calf serum. Solutions to be assayed were added 24-48 hr later. (The solutions to be assayed were sterilized by 3-min UV irradiation with a germicidal lamp rather than by filtration, since the growth factor appears to be adsorbed on Millipore filters under some conditions.) Counts of the final number of cells per dish were made at...

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Two high molecular weight growth inhibitors have been isolated from the culture medium of BSC-1 cells, epithelial cells of African green monkey kidney. The purified kidney epithelial cell growth inhibitors, at ng/ml concentrations, reversibly arrest the growth of BSC-1 cells in the GI phase of the cell cycle. Their action is selective; they are most active on BSC-1 cells, are less active as inhibitors of the growth of rat lung and human breast epithelial cells, and do not inhibit the growth of 3T3 mouse embryo fibroblasts and human skin fibroblasts in culture. Their growth inhibitory action on BSC-1 cell cultures is counteracted by epidermal growth factor or calf serum.One of the areas of ignorance that remains in our broad understanding of the control of growth of mammalian cells is the subject of endogenous growth inhibitors. The existence of a number of growth inhibitors has been postulated (1, 2), and growth-inhibitory fractions have been obtained from various mammalian cells and tissues (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). Nevertheless, there are doubts as to the significance of endogenous growth inhibitors because most of the inhibitor preparations have low specific activity, the inhibitors have been difficult to purify, and with some preparations the causes of growth inhibition have, at least in part, turned out to be trivial (15-17). There is no doubt that highly active growth inhibitors exist for mammalian cells, because interferon and corticotropin are extremely active as growth inhibitors in certain cell cultures (18,19), but the inhibitory activities of these two substances could be incidental to their other biological activities. Because of the potential significance of growth inhibitors and because purified inhibitors could be very useful in the manipulation of growth of mammalian cells, we have undertaken further studies in this area.Density-dependent regulation of growth of BSC-1 cells (African green monkey kidney epithelial cells) has been found to result in part from the accumulation of growth inhibitors in the culture medium (20). The present paper describes the isolation of high molecular weight growth inhibitors from the culture medium of these cells. The growth inhibitors have very high specific activities, approximately equal to the specific activities of interferon and corticotropin in their respective cell culture assays. The new growth inhibitors appear to be selective in the types of cells on which they act. MATERIALS AND METHODSCell Cultures. BSC-1 cell stock cultures were maintained as previously described (21) in Dulbecco's modified Eagle's medium (DME medium) (22) Preparation of Conditioned Medium. Dense BSC-1 cell cultures, used for preparation of conditioned medium, were grown in 15-cm Falcon tissue culture plates in DME medium with 1% calf serum. The medium was changed once a week until the cells reached a density of approximately 1.5 X 105 cells per cm2. The cultures were then maintained in DME medium with 0.1% calf serum. To prepare conditioned medium, the cultu...