Bacteria and archaea are engaged in a constant arms race to defend against the ever-present threats of viruses and invasion by mobile genetic elements. The most flexible weapons in the prokaryotic defense arsenal are the CRISPR-Cas adaptive immune systems, which are capable of selective identification and neutralization of foreign elements. CRISPR-Cas systems rely on stored genetic memories to facilitate target recognition. Thus, to keep pace with a changing pool of hostile invaders, the CRISPR memory banks must be regularly updated by the addition of new information, through a process termed adaptation. In this review, we outline the recent advances in our understanding of the molecular mechanisms governing adaptation and highlight the diversity between systems.

We report the first identified mutation in the gene encoding human cytochrome c (CYCS). Glycine 41, invariant throughout eukaryotes, is substituted by serine in a family with autosomal dominant thrombocytopenia caused by dysregulated platelet formation. The mutation yields a cytochrome c variant with enhanced apoptotic activity in vitro. Notably, the family has no other phenotypic indication of abnormal apoptosis, implying that cytochrome c activity is not a critical regulator of most physiological apoptosis.

In this review we examine the structure and function of the extrinsic proteins of Photosystem II. These proteins include PsbO, present in all oxygenic organisms, the PsbP and PsbQ proteins, which are found in higher plants and eukaryotic algae, and the PsbU, PsbV, CyanoQ, and CyanoP proteins, which are found in the cyanobacteria. These proteins serve to optimize oxygen evolution at physiological calcium and chloride concentrations. They also shield the Mn(4)CaO(5) cluster from exogenous reductants. Numerous biochemical, genetic and structural studies have been used to probe the structure and function of these proteins within the photosystem. We will discuss the most recent proposed functional roles for these components, their structures (as deduced from biochemical and X-ray crystallographic studies) and the locations of their proposed binding domains within the Photosystem II complex. This article is part of a Special Issue entitled: Photosystem II.

Significance CRISPR-Cas systems provide prokaryotic adaptive immunity against invading genetic elements. For immunity, fragments of invader DNA are integrated into CRISPR arrays by Cas1 and Cas2 proteins. Type I-F systems contain a unique fusion of Cas2 to Cas3, the enzyme responsible for destruction of invading DNA. Structural, biophysical, and biochemical analyses of Cas1 and Cas2-3 from Pectobacterium atrosepticum demonstrated that they form a 400-kDa complex with a Cas1 4 :Cas2-3 2 stoichiometry. Cas1–Cas2-3 binds, processes, and catalyzes the integration of DNA into CRISPR arrays independent of Cas3 activity. The arrangement of Cas3 in the complex, together with its redundant role in processing and integration, supports a scenario where Cas3 couples invader destruction with immunization—a process recently demonstrated in vivo.

The naturally-occurring G41S mutation to human (Hs) cytochrome (cyt) c enhances apoptotic activity based upon previous in vitro and in vivo studies, but the molecular mechanism underlying this enhancement remains unknown. Here, X-ray crystallography, nuclear magnetic resonance (NMR) spectroscopy, and density functional theory (DFT) calculations have been used to identify the structural and electronic differences between wild-type (WT) and G41S Hs cyt c. S41 is part of the hydrogen bonding network for propionate 7 of heme pyrrole ring A in the X-ray structure of G41S Hs cyt c and, compared to WT, G41S Hs cyt c has increased spin density on pyrrole ring C and a faster electron self-exchange rate. DFT calculations illustrate an electronic mechanism where structural changes near ring A can result in electronic changes at ring C. Since ring C is part of the solvent-exposed protein surface, we propose that this heme electronic structure change may ultimately be responsible for the enhanced proapoptotic activity of G41S Hs cyt c.In addition to its well described role in the mitochondrial electron transport chain, human (Hs) cytochrome (cyt) c is active in the intrinsic apoptotic pathway where it is released into the cytosol and interacts with apoptosis protease activating factor-1 (Apaf-1). This interaction promotes apoptosome assembly, caspase activation, and, ultimately, triggers cell death. The G41S mutant, which was previously identified in patients diagnosed with a form of mild autosomal dominant thrombocytopenia consistent with enhanced apoptotic activity in vivo, was the first mutation identified in Hs cyt c and is currently the only variant of Hs cyt c known to have an enhanced ability to activate caspases in vitro.1 The G41S mutation does not appear to affect mitochondrial respiration and the molecular mechanism underlying the enhanced caspase activation remains unknown.The in vitro assay monitors caspase activation in cytosolic extracts, which suggests that the proapoptotic activity of G41S cyt c is related to the apoptosome assembly step. A 9.5-Å resolution structure of the human apoptosome was recently reported; the density map is consistent with the presence of folded cyt c, but the binding orientation could not be determined unambiguously.2 Nevertheless, researchers have identified several charged surface residues in the vicinity of pyrrole rings A and C that are critical for both in vitro and in vivo binding and activation of Apaf-1.3 , 4 G41 is not a charged surface residue, but * bren@chem.rochester.edu . Supporting Information Available:Experimental details, NMR resonance assignments, PBE DFT-computed MOs, and complete citations for references 1 and 12. This material is available free of charge via the Internet at http://pubs.acs.org. NIH Public AccessAuthor Manuscript J Am Chem Soc. Author manuscript; available in PMC 2012 February 9. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript several groups have identified another important factor governing the interaction of ...