Regional and temporal control of genetic transcription in phage lambda

Finding, Conducting, and Nurturing Science: A Virologist's Memoir

My laboratory investigations have been driven by an abiding interest in understanding the consequences of genetic rearrangement in evolution and disease, and in using viruses to elucidate fundamental mechanisms in biology. Starting with bacteriophages and moving to the retroviruses, my use of the tools of genetics, molecular biology, biochemistry, and biophysics has spanned more than half a century-from the time when DNA structure was just discovered to the present day of big data and epigenetics. Both riding and contributing to the successive waves of technology, my laboratory has elucidated fundamental mechanisms in DNA replication, repair, and recombination. We have made substantial contributions in the area of retroviral oncogenesis, delineated mechanisms that control retroviral gene expression, and elucidated critical details of the structure and function of the retroviral enzymes-reverse transcriptase, protease, and integrase-and have had the satisfaction of knowing that the fundamental knowledge gained from these studies contributed important groundwork for the eventual development of antiviral drugs to treat AIDS. While pursuing laboratory research as a principal investigator, I have also been a science administrator-moving from laboratory head to department chair and, finally, to institute director. In addition, I have undertaken a number of community service, science-related "extracurricular" activities during this time. Filling all of these roles, while being a wife and mother, has required family love and support, creative management, and, above all, personal flexibility-with not too much long-term planning. I hope that this description of my journey, with various roles, obstacles, and successes, will be both interesting and informative, especially to young female scientists.

Little lambda, who made thee?

The study of the bacteriophage lambda has been critical to the discipline of molecular biology. It was the source of key discoveries in the mechanisms of, among other processes, gene regulation, recombination, and transcription initiation and termination. We trace here the events surrounding these findings and draw on the recollections of the participants. We show how a particular atmosphere of interactions among creative scientists yielded spectacular insights into how living things work.

Temporally Regulated Transcriptional Expression of the Genomes of Lactococcal Bacteriophages c2 and sk1

Transcription maps of the Lactococcus lactis subsp. lactis prolate phage c2 and small isometric phage sk1 were constructed. Early and late transcripts were demonstrated in phage c2. Early transcription was localized to within a 7.5-kb Eco RV fragment, and late transcription included the region which encodes the phage structural proteins and a lysin gene. Early, middle, and late transcripts were demonstrated in phage sk1. Transcription was confined to an 11.3-kb region defined by the three Eco RV restriction fragments of 6.2, 4.7, and 0.46 kb during the early part of the sk1 life cycle. Middle gene transcripts extended from the Eco RV site (defining the left-hand limit of early gene expression) through the cos site and included the 4.3-kb Pvu II- cos fragment. Late transcription was detected over the remainder of the phage genome. These results indicated that gene expression was temporally regulated at the level of transcription in these two lactococcal phages and that two regions of time-dependent transcription exist in phage c2 and three in phage sk1.

Comparisons of the distribution of nucleotides and common sequences in deoxyribonucleic acid from selected bacteriophages

Results from comparisons of deoxyribonucleic acid (DNA) from several classes of bacteriophages suggest that most phage chromosomes contain either a homogeneous distribution of nucleotides or are made up of a few, rather large segments of different quanine plus cytosine (G + C) contents which are internally homogeneous. Among those temperate phages tested, most contained segmented DNA. Comparisons of sequence similarities among segments from lambdoid phage DNA species revealed the following order in relatedness to lambda: 82 (and 434) > 21 > 424 > phi80. Most common sequences are found in the highest G + C segments, which in lambda contain head and tail genes. Hybridization tests with lambda and 186 or P2 DNA species verified that the lambdoids and 186 and P2 belong to two distinct groups. There are fewer homologous sequences between the DNA species of coliphages lambda and P2 or 186 than there are between the DNA species of coliphage lambda and salmonella phage P22.

Base sequence homology and renaturation studies of the deoxyribonucleic acid of extremely halophilic bacteria

The genetic relatedness among various strains of halophilic bacteria has been assessed by deoxyribonucleic acid-deoxyribonucleic acid (DNA-DNA) duplex formation and ribosomal ribonucleic acid (RNA) hybridization. All of the strains of extremely halophilic rods are closely related, and the extent of divergence of base sequence is similar for the major and minor DNA components. Parallel experiments with ribosomal RNA revealed a relationship between the extremely halophilic rods and cocci and a more distant relationship to moderate halophiles and to a photosynthetic extreme halophile. Renaturation studies of halophile DNA exclude the possibility that the satellite DNA represents multiple copies of a small episomal element. The kinetics of DNA renaturation show that the genome size of the extreme and moderate halophiles is similar to that of Escherichia coli.

Nucleotide distribution and functional orientation in the deoxyribonucleic acid of phage phi 80

The distribution of nucleotides in the deoxyribonucleic acid (DNA) of phi80 was determined by density analysis of molecular fragments of known length and origin. One half of the molecule includes a long, fairly homogeneous segment that contains 55% guanine plus cytosine (GC). DNA in the other half contains about 50% GC, except for a short stretch near the molecular end where the GC content is higher. Transcription of phi80 DNA was studied by ribonucleic acid-DNA hybridization tests with isolated molecular halves. At times early in the growth cycle, messenger is synthesized at a constant, relatively low rate and originates almost exclusively from DNA in the lower-GC half. At later times, messenger represents both halves and is synthesized at a greatly increased rate. The DNA of phi80 is closely analogous, physically and functionally, to lambda DNA. The similarity is most striking in the high-GC half, which is defined here as "left" since its molecular end contains a cohesive site homologous to that at lambda's left end. The left halves of the DNA of the two phages contain primarily late-functioning genes, and possess similar nucleotide distributions and some similar base sequences. The right halves of the two DNA molecules are less similar. Both contain the early genes, but they differ considerably in GC content (45% in lambda, 51% in phi80) and do not strongly cross-react in hybridization tests. The DNA of phi80 lacks the central 37%-GC segment found in lambda.

The molecular origin of lambda prophage mRNA

Lambda-specific RNA extracted from lysogenic bacteria hybridizes specifically with fragments of lambda DNA containing 43 per cent GC (guanine plus cytosine). Therefore genes known to function in the prophage state (c(I) and rex) lie 0.38 +/- 0.08 fractional molecular length from the right end of the lambda DNA molecule, according to the compositional map of Skalka, Burgi, and Hershey.

Intramolecular heterogeneity of the deoxyribonucleic acid of temperate bacteriophages

Spectral changes accompanying the thermal denaturation of phage deoxyribonucleic acid suggested that lambda is not unique in possessing large-scale intramolecular heterogeneity and nucleotide clustering; instead, lambda seems to share this property with other enteric phages.