The mechanism by which phenytoin (PHT) induces gingival overgrowth remains unclear. We hypothesized that PHT increases macrophage production of platelet-derived growth factor (PDGF), an important cytokine in connective tissue growth and repair, and that excessive production PDGF in gingiva could lead to redundant growth. To test the hypothesis, rat peritoneal macrophages and human blood monocytes were cultured in the presence of PHT (5 to 20 micrograms/ml medium) or an equal volume of its solvent for 3 days and tested for expression of PDGF-B mRNA by in situ hybridization. Approximately 300 cells/culture well were examined (3 wells/drug level) for positive indication of PDGF-B mRNA. Data were compared by chi square test. All levels of PHT in both cell types induced a 2- to 8-fold increase in PDGF-B mRNA positive cells, significant in all cases at P < 0.001. Northern blot analysis of RNA from similarly cultured rat macrophages confirmed these findings. Cells treated with 10 micrograms PHT/ml medium or solvent revealed 2.2 +/- 0.3 and 1.0 +/- 0.2 (mean +/- SEM) arbitrary units PDGF mRNA respectively (t tests, P < 0.05). Additionally, rat macrophages were cultured in presence of 5 micrograms PHT/medium or its solvent and medium was analyzed for PDGF secretion by radioimmunoassay. Mean values (+/- SEM) were 1.28 +/- 0.49 and 0.78 +/- 0.07 ng/mg protein respectively (t test, P < 0.05). These data showed that PHT augmented the expression of c-sis, the gene for PDGF-B, and offered a possible explanation for PHT-induced gingival overgrowth.

The effect of the new antibiotic, myxin, on the syntheses of deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and protein in Escherichia coli (strains B and 15T-) was examined. Within 7 min of the addition of myxin at 5 ,ug/ml, the synthesis of new bacterial DNA was almost completely inhibited. This was followed by an extensive degradation of the pre-existing DNA to an acid-soluble form. All of the evidence indicated that the primary effect of the antibiotic was on cellular DNA. The synthesis of RNA was completely inhibited after 15 min of exposure to myxin (5 ,g/ml), and the synthesis of protein was markedly reduced after 30 min. There was no measurable breakdown of either RNA or protein in the myxin-treated cells. A marked stimulation of "4C-uracil incorporation was found in the presence of myxin in 15Tcells only. This did not result from an increased rate of RNA synthesis but was due to an increase in the proportion of exogenous uracil, relative to endogenous uracil, incorporated into cellular RNA. This probably reflected a partial inhibition of the biosynthesis of uridine monophosphate from orotate. At 4.5 ,Ag of myxin per ml and with 0.8 X 108 cells per ml, 50% of the antibiotic was reduced in 15 min from the biologically active oxidized form to the biologically inactive state. Under these conditions, a maximum of 0.6% (27 u/Ag/ml) of the myxin was retained in the cells.

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Fifteen specific bacteriophages, each active on particular strains of Rhizobium meliloti, have been selected from those isolated by enrichment of local soils. Variables affecting phage–host interaction have been examined and standardized. The differential susceptibility of individual R. meliloti strains to each phage produces a distinctive pattern of response which allows the segregation of each strain into one of the 80 different groups identified to date. Discrimination between strains with this typing system is reproducible with no change in phage type following extended subculture or plant infection and reisolation of strains from root nodules of alfalfa. The large number of different strains recognized by the system should make it very useful in experimentally controlled tests of selected strains in the nodulation of alfalfa plants.

T2r+ bacteriophage grown in its host, Escherichia coli B, in broth medium in the presence of radioactive inorganic phosphorus was labelled with the isotope. Purified suspensions of this virus had specific activities up to 50,000 c.p.m. per μgm. P. There was little or no exchange of P32 between virus and inorganic phosphate. Chemical analysis showed that at least 98% of the virus phosphorus was contained in nucleic acid; of the nucleic acid phosphorus 95.5% was associated with desoxypentose nucleic acid and 4.5% with pentose nucleic acid. More than 99% of the radioactivity of the labelled bacteriophage was contained in the nucleic acid fraction. Preparations of bacteriophage were obtained with sufficiently high specific activity to enable metabolism experiments to be carried out on the growth of the labelled virus in the host cell.

Leaf discs from 15 mutant clones of tomato were tested for their morphogenetic response in Murashige and Skoog medium supplemented with 12 combinations of the growth regulators napthaleneacetic acid (NAA) and benzylaminopurine (BA) and 4 combinations of NAA and zeatin. The results show that either callus, shoots, roots, or shoots and roots can be produced depending upon the hormone concentrations and ratios. Plants were regenerated from 12 of the 15 varieties tested.

Exposure of Escherichia coli 15T-cells to the antibiotic myxin results in the inhibition of deoxyribonucleic acid (DNA) biosynthesis, degradation of intracellular DNA, and death of the cells. Each of these effects was markedly enhanced when protein synthesis was simultaneously inhibited by chloramphenicol. In the continued presence of chloramphenicol, a brief (1 min) exposure to myxin resulted in a rate of DNA degradation and cell death equivalent to that found in the continued presence of myxin alone. Single-strand breaks were present in the DNA of cells exposed to myxin, but when chloramphenicol was also present the breaks were found much earlier. Degradation of DNA in cells exposed to myxin was found to be distributed randomly in both strands and extended over the genome with no restriction to the vicinity of the replication point. There was no release of DNA from its attachment to the cellular membrane in myxinexposed cells. The possibility that the chloramphenicol effect is due to the inhibition of repair enzyme synthesis which is stimulated by exposure of the cells to myxin is discussed. These data indicate that the extent of the lethal and metabolic damage to the cells by an exposure to myxin represents the result of competition between damage to and repair of cellular DNA. was dissolved in M9 medium. "4C-labeled myxin was prepared as previously reported (17). Nalidixic acid was obtained from Calbiochem, Los Angeles, Calif., and chloramphenicol, from Sigma Chemical Co., St. Louis, Mo. Radioactive materials. Thymine-2-4C, uridine-2-14C, thymine-6-3H and uniformly labeled '4C-Lleucine were obtained from New England Nuclear Corp., Boston, Mass. Synthesis of macromolecules. DNA and protein biosynthesis was followed by measuring the incorporation of radioactive thymine and leucine, respectively, into the cold trichloroacetic acid-insoluble fraction of the cells, as described previously (16). In experiments where cells containing uniformly labeled DNA were required, the cells were grown in the presence of '4C-thymine for more than four gen-250

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