Millard Susman

STUDIES ON PHAGE DEVELOPMENT. I. AN ACRIDINE-SENSITIVE CLOCK.

Abstract Bacteria infected with T4 bacteriophage do not produce phage in the presence of 9-aminoacridine (9AA). The acridine inhibits both maturation and lysis. Pulse treatments with 9AA show that the production of infective phage depends on a continuous, acridine-sensitive process that begins 7 minutes after infection (at 30°C) and that culminates at 23 minutes in the production of the first mature phage. The presence of 9AA at any time during the critical period between 7 and 23 minutes leads to a corresponding delay in maturation after the inhibitor is removed. This acridine-sensitive process that affects the timing of maturation has been called the maturation clock . In cells in which infective intracellular phage has begun to appear, maturation stops within 1 minute after addition of 9AA. The inhibition of lysis depends on the time at which 9AA is added. Maximum inhibition is achieved by adding 9AA within the first 10 minutes of the latent period. The dye no longer has any effect on lysis if the cells are first exposed to the dye at 20 minutes after infection. Evidence is presented to show that 9AA does not prevent the synthesis of phage DNA or phage protein. Furthermore, the maturation clock does not appear to depend on the synthesis of phage DNA or protein.

Studies on phage development: III. The fate of T4 DNA and protein synthesized in the presence of 9-aminoacridine

9-Aminoacridine (9AA) inhibits maturation of phage T4. Upon diluting infected bacteria from the dye into a medium containing an inhibitor of protein synthesis, one obtains a crop of mature phage made out of the material accumulated before the removal of 9AA. DNA is synthesized in the presence of 9AA and continues to replicate after the removal of 9AA and addition of puromycin. By marking the DNA with 5-bromodeoxyuridine (BUDR) and studying the light sensitivity of the phage produced by cells exposed to 9AA, BUDR and puromycin in various programs, one can show that the DNA made in the presence of 9AA is incorporated later into viable phage. This early DNA is dispersed through the DNA synthesized after the removal of the dye. The DNA can in the presence of puromycin pass from the vegetative into the condensed state and subsequently be coated with the protein material synthesized in the presence of the dye.

Studies on phage development. II. The maturation of T4 phage in the presence of puromycin.

Maturation of T4 can proceed in the presence of 5-methyltryptophan or puromycin. Puromycin inhibits phage protein synthesis within a few seconds, thus limiting the amount of phage precursor material available for maturation. After the arrest of protein synthesis, maturation continues unabated until the limiting protein precursor is depleted. At least one protein, the tail fiber protein, is depleted. Phage protein maturable in the presence of puromycin appears about 5 minutes before maturation begins. The level of the maturable material reaches a maximum at the beginning of maturation. This level is taken as a measure of the size of the pool of complete sets of maturable phage protein. The bulk precursor protein begins to accumulate 3 minutes earlier than serum blocking proteins (SBP) and forms a pool about twice as large as the pool of SBP.

A centennial: George W. Beadle, 1903-1989.

GEORGE BEADLE was a quadruple-threat man—scientist, teacher, administrator, and public citizen. He excelled in each. Furthermore, he did what very few geneticists did in his time: he studied three different organisms and made outstanding discoveries in every case. He followed his interests and

James F. Crow (1916–2012)

A population geneticist is remembered by colleagues for his generosity, clarity, and influence in the field, as well as in policy matters involving genetics. James Franklin Crow pioneered population genetics and influenced policy in human genetics. He was a scholar, citizen, and friend, who touched the lives of students, colleagues, musicians, and many others. He was born on 18 January 1916 in Phoenixville, Pennsylvania. After receiving his bachelors degree from Friends University in 1937 and his Ph.D. in genetics from the University of Texas at Austin in 1941, he joined the faculty at Dartmouth College. In 1948, he moved to the University of Wisconsin–Madison, where he remained fully active until his death on 4 January 2012 at the age of 95.

James F. Crow: storied teacher, leader, and colleague at the University of Wisconsin.

ALL told, the two of us knew James F. Crow for 105 years. Rayla Greenberg Temin received her Ph.D. in genetics in 1963 with Jim as her major professor. After receiving her degree, Rayla remained in Jim’s lab for many years as a research scientist working in collaboration with Jim. In 1978 she

Sister Chromatid Exchanges and Mitotic Crossing-Over

Sister chromatid exchanges (SCE) were discovered in the late 1950s with the same type of experiment as that which demonstrated the semiconservative replication of chromosomes (Chapter III). When cells are grown for one cycle in medium containing 3H-thymidine and for another without it, autoradiography shows that one chromatid is labeled and the other is not (Figs. III.2, III.3); this makes exchanges between them visible (Taylor, 1958).

Architecture and Function of the Eukaryotic Chromosome

Viewed from a molecular perspective, the eukaryotic chromosome is an immense structure. Viewed from a physiological perspective, it is not one structure at all, but a family of related structures, differing from one another in form and activity. Even a single chromosome has pronounced structural variation along its length and is changeable from one moment to the next. These factors make the chromosome a difficult object to study, and our understanding of the details of its structure and function remains incomplete. However, in recent years the analytical methods of molecular biology and the discovery of model systems in which to study chromosome function have greatly expanded our understanding of the eukaryotic chromosome. The results have been reviewed in numerous books and articles (for example, Alberts et al., 1983, 1989; Darnell et al., 1990; Jeppesen and Bower, 1987).

Chromosome Arrangement in Interphase and in Differentiated Nuclei

Until some 10 years ago our knowledge of the structure of interphase nuclei was limited to a few aspects of chromosome arrangement. As early as 1885, Carl Rabl showed that chromosomes remain in the same polarized orientation through interphase that they had assumed in anaphase. This Rabl orientation, as it has been called, is often still visible in the following prophase (Fig. 14.1a, b; Heitz, 1933). Rabl also found that chromosomes in interphase do not form a tangle of threads, but that each of them occupies a defined territory, a domain. Recent studies done by creating prematurely condensed chromosomes (Sperling and Ludtke, 1981; Cremer et al., 1982) and by hybridizing specific DNA probes to nuclei (Manuelidis, 1985; Schardin et al., 1985) have shed new light on the nuclear structure. The latter technique, especially, has made visible the domains of individual chromosomes and has enabled the determination of numerical and structural chromosome abnormalities in interphase nuclei (Cremer et al., 1988).

Abnormal Human Sex Chromosome Constitutions

The sex chromosomes show a much wider range of viable aneuploidy than do the autosomes, presumably for the following reasons. On the one hand, the Y chromosome seems to contain very few genes apart from those determining the male sex; on the other, all but one X chromosome in a cell are inactivated, forming X chromatin bodies in the interphase. This rule can be stated another way: there is one active X chromosome for each diploid complement of autosomes.