In both natural and engineered biological systems, RNA-guided proteins have emerged as critical transcriptional regulators by modulating RNA polymerase (RNAP) and its associated factors1-3. In bacteria, diverse clades of repurposed TnpB and CRISPR-associated proteins repress gene expression by blocking transcription initiation or elongation, enabling non-canonical modes of regulatory control and adaptive immunity1,4,5. A distinct class of nuclease-dead Cas12f homologues (dCas12f) instead activates gene expression through its association with unique extracytoplasmic function sigma factors (σE)6, although the molecular basis has remained elusive. Here we reveal a new mode of RNA-guided transcription initiation by determining the cryo-electron microscopy structures of the dCas12f-σE system from Flagellimonas taeanensis. We captured multiple conformational and compositional states, including the DNA-bound dCas12f-σE-RNAP holoenzyme complex, revealing how RNA-guided DNA binding leads to σE-RNAP recruitment and nascent mRNA synthesis at a precisely defined distance downstream of the R-loop. Rather than following the classical paradigm of σE-dependent promoter recognition, these studies show that recognition of the -35 element is largely supplanted by CRISPR-Cas targeting, whereas the melted -10 element is stabilized through unusual stacking interactions rather than insertion into the typical recognition pocket. Collectively, this work provides high-resolution insights into an unexpected mechanism of RNA-guided transcription, expanding our understanding of bacterial gene regulation and opening new avenues for programmable transcriptional control.

Insects make up the majority of all animal species, with 70% occurring in the tropics1, yet the impacts of warming on tropical insects remain highly uncertain2. This stems from sparse, taxonomically biased data on thermal tolerance of tropical insects and an incomplete understanding of the underlying physiological mechanisms3. Here we compared environmental temperatures with field-measured upper and lower thermal tolerance limits of around 2,300 insect species along Afrotropical and Neotropical elevational gradients and identified genomic signatures of thermal tolerance across the insect tree of life. We show that thermal tolerances do not proportionally track environmental temperatures but approach an asymptote in tropical lowlands. Insects at high elevations utilize plasticity to cope with rising temperatures, whereas lowland species have limited plastic abilities. Heat tolerance showed strong differences among insect orders and families, reflected in the thermal stability of proteins, suggesting that variation in thermal tolerance is founded in the fundamental protein architecture. Up to 52% of future surface temperatures and 38% of air temperatures in the Amazonian lowlands can cause heat mortality in half of the studied community. Our data suggest a limited capacity of insects in the Earth's most biodiverse regions to buffer future warming.