TRANSCRIPTION OF A REPRESSED GENE: EVIDENCE THAT IT REQUIRES DNA REPLICATION

It is known that the synthesis of DNA is not essential for constitutive or induced enzyme synthesis. Our studies indicate that DNA synthesis is required for the basal level synthesis (that is, for synthesis in absence of inducer) of two inducible enzymes, a finding which supports an earlier speculation that a messenger-RNA transcription event may normally accompany DNA replication. Studies with cultures of Escherichia coli TAU-bar in which DNA replication is synchronized suggest that the beta-galactosidase gene is transcribed at a particular time in the sequential replication of the bacterial chromosome.

SPECIFICITY OF POTASSIUM-ACTIVATED PHOSPHODIESTERASE OF ESCHERICHIA COLI

A potassium-activated phosphodiesterase that hydrolyzes polyribonucleotides to 5'-mononucleotides has been purified approximately 600-fold from extracts of Escherichia coli B. The purified enzyme appears to be specific for single-stranded polyribonucleotides: helical forms are not hydrolyzed, nor do they inhibit the hydrolysis of single-stranded chains.

THE BIOSYNTHESIS OF POLYSACCHARIDES. INCORPORATION OF D-(1-14C)GLUCOSE AND D-(6-14C)GLUCOSE INTO PLUM-LEAF POLYSACCHARIDES

1. The utilization of specifically labelled d-glucose in the biosynthesis of plum-leaf polysaccharides has been studied. After these precursors had been metabolized in plum leaves, the polysaccharides were isolated from the leaves, and their monosaccharide constituents isolated and purified. 2. Both the specific activities and the distribution of (14)C along the carbon chains of the monosaccharides were determined. Significant (14)C activity was found in units of d-galactose, d-glucose, d-xylose and l-arabinose, but their specific activities varied widely. The labelling patterns suggest that in the leaves the other monosaccharides all arise directly from d-glucose without any skeletal change in the carbon chain, other than the loss of a terminal carbon atom in the synthesis of pentoses. 3. The results indicated that within the leaf there are various precursor pools for polysaccharide synthesis and that these pools are not in equilibrium with one another.

UPTAKE AND INCORPORATION OF THYMINE, THYMIDINE, URACIL, URIDINE, AND 5-FLUOROURACIL INTO THE NUCLEIC ACIDS OF BACILLUS SUBTILIS

Bodmer, Walter F. (Stanford University School of Medicine, Palo Alto, Calif.), and Susan Grether. Uptake and incorporation of thymine, thymidine, uracil, uridine, and 5-fluorouracil into the nucleic acids of Bacillus subtilis. J. Bacteriol. 89:1011-1014. 1965.-From 55 to 95% of uracil, uridine, or 5-fluorouracil (FU) added to the culture medium is incorporated into the acid-insoluble fraction of cells of Bacillus subtilis strains SB 19 (prototroph) and SB 503 (FU-resistant). Thymine is poorly incorporated (less than 1%); thus, the incorporation of thymidine is limited (less than 12%) by the rapid degradation of the nucleoside to thymine, probably by the enzyme thymidine phosphorylase. Uracil, uridine, and FU were not incorporated into the deoxyribonucleic acid (DNA) of either strain, whereas all the incorporated thymidine was found in the DNA.

USE OF S-METHYLCYSTINE AND CYSTATHIONINE BY METHIONINELESS NEUROSPORA MUTANTS

Wiebers, Joyce L. (Purdue University, West Lafayette, Ind.), and Harold R. Garner. Use of S-methylcysteine and cystathionine by methionineless Neurospora mutants. J. Bacteriol. 88:1798-1804. 1964.-Radioactive methionine was found in hydrolysates of various strains of Neurospora crassa when either S-methylcysteine (SMC)-C(14)H(3) or SMC-S(35) is the sole addition to minimal medium. Isotope product-precursor specific activity ratios are very similar for the two sources of label. Wild-type and methionineless mutants use sulfur from SMC in the biosynthesis of methionine, but not of cysteine, when grown in regular medium. With a medium nearly free from sulfate, SMC served as a source of sulfur for both cysteine and methionine. Suppressed methionineless mutants incorporated sulfur from SMC into cellular cysteine even in the presence of normal amounts of sulfate. SMC as a possible metabolic precursor of methionine was compared to cystathionine in an experiment with wild-type Neurospora. The four sources of label used were: SMC-C(14)H(3), SMC-S(35), cystathionine-U-C(14), and cystathionine-S(35). In each flask, the organism was offered one of the labeled compounds plus an equivalent amount of the other compound without label. The amount of each compound was sufficient for either to supply its contribution to all of the cellular methionine, if it were successful in competing with endogenous sources. To avoid adaptive breakdown of substrates, the compounds were added continuously at a rate consistent with the amount of growth present. The ratio of specific activity of cellular methionine to precursor was determined for each labeled compound. The results show that SMC sulfur and methyl carbon are used equally well. Cystathionine carbon and sulfur appear to be equally utilized also. A preference for cystathionine is indicated.