Changes in histone H3 composition and synthesis pattern during lymphocyte activation

Freshly isolated human lymphocytes were found to synthesize histones at a significant rate even though no DNA was being synthesized. The synthesis pattern of histone variants in resting lymphocytes is similar to that found in other quiescent cells and different from that found in S-phase cells. For this reason, the histone synthesis in resting lymphocytes cannot be attributed to contamination by S-phase cells. Stimulation by the mitogen phytohemagglutinin resulted in a dramatic switch in the histone H3 variant synthesis pattern as well as a readily apparent change in the histone H3 mass pattern. Thus, the chromatin of activated lymphocytes has a different histone H3 variant composition than resting or quiescent lymphocytes. It is suggested that the proportion of H3.3 in the mass pattern of the chromatin of a cell may be related solely to how long that cell has been quiescent. Inducing resting lymphocytes to synthesize DNA by UV irradiation did not qualitatively change the histone variant synthesis pattern. No S-phase H3 variants were induced by the repair process. Furthermore, the quantity of histone synthesized neither increased nor decreased after treatment with UV light.     

Partial characterization of the phosphotransferase system of human central-nervous-system myelin.

The phosphotransferase system of human central-nervous-system myelin was investigated. Evidence obtained indicated the presence of at least two different phosphotransferase systems (cyclic nucleotide-dependent and -independent) in myelin, which were found to be firmly associated with the membrane. The cyclic AMP-dependent kinase of myelin and white-matter cytosol preferentially phosphorylated certain histone fractions and displayed only modest activity with basic protein as substrate. On the other hand, the cyclic nucleotide-independent system showed specificity toward basic protein. Its activity was not only dependent on Mg2+ but it was greatly enhanced by this bivalent cation. Whereas the cyclic nucleotide-dependent kinase could be extracted with buffers containing Triton X-100, the bivalent cation-regulated kinase resisted solubilization from myelin under these conditions.     

Expression of Cryptic β-Fructofuranosidase in Saccharomyces rouxii

Raffinose hydrolysis was studied in Saccharomyces rouxii. The responsible enzyme was identified as a beta-fructofuranosidase (EC 3.2.1.26), which has a pH optimum of 5.5 and a K(m) of 83 mM for raffinose. This enzyme was cryptic in cells from a 3-day culture. A 2-min treatment with 0.1 volume of ethyl acetate in sodium acetate buffer (pH 6) gave complete expression of the enzyme, which was still retained by the cell. Ghosts were prepared by modifying membrane structure with small basic proteins in distilled water, and after washing they showed the full complement of enzymatic activity. The enzyme remained cryptic in osmotically protected spheroplasts; however, after lysis (by dilution) release, as well as expression, was effected. Mechanical disruption of fresh cells revealed and released all of the enzyme. Cells in early stationary phase had all of their beta-fructofuranosidase in a cryptic state. Aging yielded fractional expression of activity; initially this was proportional to cell death, but later the degree of expression exceeded the death rate. Media from aged cultures or cell-free extracts of aged cells were not effective in revealing the cryptic enzyme of younger cells. S. rouxii beta-fructofuranosidase has a different autolytic-release pattern from its counterpart in S. cerevisiae. Also, high concentrations of glucose do not repress the S. rouxii enzyme. The beta-fructofuranosidase in young cells of S. rouxii must be enclosed by the protoplasmic membrane or a special vesicular structure. This system was compared with other Saccharomyces species in connection with the translocation of enzymes across the protoplasmic membrane.     

A model for the tertiary structure of transfer ribonucleic acids

In view of the possible changes of the structure of DNA at different relative humidities, a similar model for tRNA is proposed. In the native form, the sugar phosphate backbone folds repeatedly on itself resulting in a circular rod structure of about 50 Å in length and about 25 Å in diameter. Both the amino-acyl end and the anticodon site are surrounded by segments of base sequences common to different tRNA’s.