Target RNA-guided protease activity in type III-E CRISPR-Cas system.

The type III-E CRISPR-Cas systems are newly identified adaptive immune systems in prokaryotes that use a single Cas7-11 protein to specifically cleave target RNA. Cas7-11 could associate with Csx29, a putative caspase-like protein encoded by the gene frequently found in the type III-E loci, suggesting a functional linkage between the RNase and protease activities in type III-E systems. Here, we demonstrated that target RNA recognition would stimulate the proteolytic activity of Csx29, and protein Csx30 is the endogenous substrate. More interestingly, while the cognate target RNA recognition would activate Csx29, non-cognate target RNA with the complementary 3' anti-tag sequence inhibits the enzymatic activity. Csx30 could bind to the sigma factor RpoE, which may initiate the stress response after proteolytic cleavage. Combined with biochemical and structural studies, we have elucidated the mechanisms underlying the target RNA-guided proteolytic activity of Csx29. Our work will guide further developments leveraging this simple RNA targeting system for RNA and protein-related applications.

Gain analysis of optically-pumped Si nanocrystal waveguide amplifiers on silicon substrate.

The SiO(2)/SiO(x)/SiO(2) strip-loaded waveguide on Si substrate with buried Si nanocrystals (Si-ncs) in SiO(x) layer is demonstrated to show the Si-nc dependent optical gain. The amplified spontaneous emission (ASE) spectrum at 750-850 nm is observed with central wavelength of 805 nm and 3dB spectral linewidth of 140 nm. The optical net modal gain and loss coefficients of 85.7 cm(-1) and 21 cm(-1), respectively, are determined from the waveguide length dependent ASE intensity. By attenuating 785-nm laser diode signal to inject the pumped SiO(2)/SiO(x)/SiO(2) strip-loaded waveguide, a small-signal power gain of 13.5 decibel (dB) is obtained. Increasing the laser diode power shows a significantly reduced power gain with a saturated output power due to the finite density of the optically pumped Si-ncs. The fitting of power-dependent gain with a gain-saturated amplifier model reveals a peak gain of 35 dB and a saturation power of 1.1 nW for the SiO(2)/SiO(x):Si-nc/SiO(2)/Si strip-loaded waveguide. Similar output saturation is also observed with increasing pumping power. With the presence of optical gain in the optically pumped Si-ncs, the intended application will be the monolithic integration of the Si-nc based optical waveguide amplifier with the other on-board photonic integrated circuits for the future optical interconnect communication.