Hybrid Vigor: Importance of Hybrid λ Phages in Early Insights in Molecular Biology

Laboratory-generated hybrids between phage λ and related phages played a seminal role in establishment of the λ model system, which, in turn, served to develop many of the foundational concepts of molecular biology, including gene structure and control. Important λ hybrids with phages 21 and 434 were the earliest of such phages. To understand the biology of these hybrids in full detail, we determined the complete genome sequences of phages 21 and 434. Although both genomes are canonical members of the λ-like phage family, they both carry unsuspected bacterial virulence gene types not previously described in this group of phages. In addition, we determined the sequences of the hybrid phages λ imm21, λ imm434, and λ h434 imm21. These sequences show that the replacements of λ DNA by nonhomologous segments of 21 or 434 DNA occurred through homologous recombination in adjacent sequences that are nearly identical in the parental phages. These five genome sequences correct a number of errors in published sequence fragments of the 21 and 434 genomes, and they point out nine nucleotide differences from Sanger's original λ sequence that are likely present in most extant λ strains in laboratory use today. We discuss the historical importance of these hybrid phages in the development of fundamental tenets of molecular biology and in some of the earliest gene cloning vectors. The 434 and 21 genomes reinforce the conclusion that the genomes of essentially all natural λ-like phages are mosaics of sequence modules from a pool of exchangeable segments.

Phage Genetic Engineering Using CRISPR⁻Cas Systems

Since their discovery over a decade ago, the class of prokaryotic immune systems known as CRISPR⁻Cas have afforded a suite of genetic tools that have revolutionized research in model organisms spanning all domains of life. CRISPR-mediated tools have also emerged for the natural targets of CRISPR⁻Cas immunity, the viruses that specifically infect bacteria, or phages. Despite their status as the most abundant biological entities on the planet, the majority of phage genes have unassigned functions. This reality underscores the need for robust genetic tools to study them. Recent reports have demonstrated that CRISPR⁻Cas systems, specifically the three major types (I, II, and III), can be harnessed to genetically engineer phages that infect diverse hosts. Here, the mechanisms of each of these systems, specific strategies used, and phage editing efficacies will be reviewed. Due to the relatively wide distribution of CRISPR⁻Cas systems across bacteria and archaea, it is anticipated that these immune systems will provide generally applicable tools that will advance the mechanistic understanding of prokaryotic viruses and accelerate the development of novel technologies based on these ubiquitous organisms.

Grete Kellenberger-Gujer: Molecular biology research pioneer

Grete Kellenberger-Gujer was a Swiss molecular biologist who pioneered fundamental studies of bacteriophage in the mid-20^th century at the University of Geneva. Her life and career stories are reviewed here, focusing on her fundamental contributions to our early understanding of phage biology via her insightful analyses of phenomena such as the lysogenic state of a temperate phage (λ), genetic recombination, radiation's in vivo consequences, and DNA restriction-modification; on her creative personality and interactions with peers; and how her academic advancement was affected by gender, societal conditions and cultural attitudes of the time. Her story is important scientifically, putting into perspective features of the scientific community from just before the molecular biology era started through its early years, and also sociologically, in illustrating the numerous “glass ceilings” that, especially then, often hampered the advancement of creative women.