Joshua Modell

Joshua Modell

Assistant Professor
Primary Appointment: 
Molecular Biology and Genetics

725 N Wolfe Street
Baltimore, MD 21205

Research topic: 

Molecular mechanisms of CRISPR-Cas Immunity

CRISPR-Cas loci serve as acquired immune systems which protect their bacterial and archaeal hosts from viruses and plasmids. CRISPR immunity occurs in two phases: First, short DNA segments from foreign invaders are captured and inserted into the CRISPR locus where they serve as DNA-based molecular memories. Second, these memories are transcribed into guide RNAs that direct CRISPR-associated nucleases to cleave matching sequences upon reinfection, thus providing immunity for the prokaryotic host. This programmable, sequence-specific cleavage activity forms the basis for CRISPR genome-editing technologies, but much remains unknown about how CRISPR systems function in their native contexts during a live bacteriophage infection.

My laboratory uses an integrated approach, combining molecular biology, next-generation sequencing, functional genomics, biochemistry, bioinformatics, and time-lapse microscopy to study both phases of immunity in type II CRISPR systems, which encode Cas9 and its homologs. We explore fundamental and mechanistic questions at the interface of CRISPR, bacterial and bacteriophage biology. Specifically, how do CRISPR systems cooperate with the biology of their bacterial hosts to maximize efficacy and prevent auto-immunity? How does the biology of a bacteriophage determine whether it will be sensitive or resistant to a CRISPR attack? And given that CRISPR systems are frequently horizontally transferred between bacterial species, what forces determine the host range and more broadly drive CRISPR evolution? By synthesizing genetic, genomic and cellular analyses, we seek to better understand the basic biology of CRISPR systems and in doing so inform the next generation of CRISPR-based technologies.​

Selected Publications: 

Goldberg, GW, McMillan EA, Varble A, Modell JW, et al. (2018) Incomplete prophage tolerance by type III-A CRISPR-Cas systems reduces the fitness of lysogenic hosts. Nat Commun. 9(1):61. Pub Med

Modell JW, Jiang W, Marraffini LA. (2017) CRISPR-Cas systems exploit viral DNA injection to establish and maintain adaptive immunity. Nature 544(7648):101-104. Nature

Heler R*, Samai P*, Modell JW, Weiner C, Goldberg GW, Bikard D, Marraffini LA. (2015) Cas9 specifies functional viral targets during CRISPR-Cas adaptation. Nature. 519(7542):199-202. Pub Med

Modell JW, Kambara TK, Laub MT. (2014) A dna damage-induced, SOS-independent checkpoint prevents cell division in Caulobacter crescentus. PLoS Biology. 12(10):e1001977. PLOS