The population and evolutionary dynamics of adaptive immunity in bacteria: the upside and downside of CRISPR-Cas
Friday seminar by Bruce R. Levin from Emory University, Atlanta, Georgia, USA. Note the time!
Arguable, surely to those who work on it, the demonstration that the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) regions and their associated sequences (Cas) that abound in the genomes of the majority of Archaea and nearly half the Bacteria can serve as an adaptive immune system has been the single most important new finding in Microbiology and Molecular Biology this millennium. By incorporating 30 or so base pairs of the infecting DNA into regions between these palindromic repeats, bacteria become immune to subsequently infection. When infected by DNAs with sequences identical to that of the incorporated “spacers”, through a process mediated by small RNAs, CRISP-Cas encoding bacteria and archaea abort the infection. When these infectious DNAs are those of lethal viruses (lytic bacteriophage) the advantages of CRISPR-Cas mediated immunity are straightforward and easily demonstrated experimentally as well as theoretically. But not all infectious DNAs are deleterious; some like plasmids may bear genes, such as those conferring resistance in the presence of antibiotics that can be very much to the advantage of recipient bacteria. In this talk I will present the results of the mathematical and computer simulation modeling and population and evolutionary dynamic and experiments and sequences analyses we have been doing to explore the upside of CRISPR-Cas immunity, protection against infections against lytic phage, and the downside of this immune system, preventing the acquisition of plasmids bearing beneficial genes. I will consider the nature of and factors limiting the CRISPR-Cas mediated Lamarckian - Darwinian arms races between bacteria and phage, and present a hypothesis to account for the extraordinary diversity in the existence, number and function of CRISPR-Cas systems within and between species of bacteria and archaea.
Bruce R. Levin,
Atlanta, Georgia 30322