Eugene Rosenberg

DNA cycle of Myxococcus xanthus

The rate of DNA synthesis during the division cycle of Myxococcus xanthus growing on a defined medium has been determined. Non-synchronized exponential phase cultures were pulse-labeled with radioactive thymidine and then prepared for quantitative autoradiography. Measurements of both cell size and grain number were performed on 2400 cells. Analysis of the distribution of grains in each of 20 size groups showed that: (a) cells do not synthesize any DNA during 20% of the division cycle; (b) those cells that are synthesizing DNA are doing so at a constant rate independent of cell size or cell age; and (c) the DNA cycle extends from 0.02 to 0.81 generation, septum formation occurs at 0.90 generation, and physical separation of sister cells at 1.0 generation. Chemical analysis of the average DNA content of M. xanthus growing in the defined medium yielded 14 × 10−9 μg DNA per cell, corresponding to 1.7 ± 0.2 chromosomes/cell. Thus, the rate of DNA synthesis reported here, 2.5 × 104 base pairs/minute, is the rate of synthesis of one replicating chromosome.

Purine and Pyrimidines in Sediments from the Experimental Mohole

Cytosine, adenine, guanine, and thymine, but not uracil, have been detected in core samples taken from various depths of the experimental Mohole. The following quantities of bases were found in the deepest core samples available (approximately 25 x 106 years old): cytosine, 2.5 �g/5 g; adenine, 0.9 �g/5 g; guanine, 0.2 �/5 g; thymine and uracil, 0 �g/5 g.

THERMAL TRANSITION OF SPIRILLUM SERPENS FLAGELLA.

The dissociation of flagella of Spirillum serpens has been studied turbidimetrically as a continuous function of temperature by following the conversion of ordered flagella to the protein subunits. Agents known to denature proteins have been found to dissociate the organelle.

Observations on the regulation of DNA biosynthesis.

Abstract It has been established that bacteria in stationary phase do not incorporate radioactive thymidine into DNA ( see Lark, 1963 ). Three of the possible explanations for the failure to incorporate thymidine in stationary phase have been considered: (1) inactive DNA polymerase, (2) depletion of one or more of the deoxyribonucleoside triphosphates, and (3) non-functionality of DNA as a primer. Experiments with E. , coli starved of an essential amino acid ( Billen, 1962m ) and the slime mold P. , polycephalium ( Guttes and Guttes, 1961 ) were interpreted to indicate that factors other than the enzyme and substrate were responsible for the regulation of DNA synthesis. This communication reports on the relative inability of DNA in sonicates of stationary phase Myxococcus , xanthus to function as an active primer for DNA synthesis.