Clustered, regularly interspaced, short palindromic repeat (CRISPR) loci and their associated (Cas) proteins provide adaptive immunity against viral infection in prokaryotes. Upon infection, short phage sequences known as spacers integrate between CRISPR repeats and are transcribed into small RNA guides that identify the viral targets (protospacers) of the Cas9 nuclease. Streptococcus pyogenes Cas9 cleavage of the viral genome requires the presence of an NGG protospacer adjacent motif (PAM) sequence immediately downstream of the target. It is not known if and how viral sequences with the correct PAM are chosen as new spacers. Here we show that Cas9 specifies functional PAM sequences during spacer acquisition. The replacement of cas9 with alleles that lack the PAM recognition motif or recognize an NGGNG PAM eliminated or changed PAM specificity during spacer acquisition, respectively. Cas9 associates with other proteins of the acquisition machinery (Cas1, Cas2 and Csn2), presumably to provide PAM-specificity to this process. These results establish a new function for Cas9 in the genesis of the prokaryotic immunological memory.

tRNA modifications are crucial for efficient and accurate protein synthesis, and modification defects are frequently associated with disease. Yeast trm7Δ mutants grow poorly due to lack of 2'-O-methylated C32 (Cm32) and Gm34 on tRNAPhe, catalyzed by Trm7-Trm732 and Trm7-Trm734 respectively, which in turn results in loss of wybutosine at G37. Mutations in human FTSJ1, the likely TRM7 homolog, cause non-syndromic X-linked intellectual disability (NSXLID), but the role of FTSJ1 in tRNA modification is unknown. Here we report that tRNAPhe from two genetically independent cell lines of NSXLID patients with loss of function FTSJ1 mutations nearly completely lacks Cm32 and Gm34, and has reduced peroxywybutosine (o2yW37). Additionally, tRNAPhe from an NSXLID patient with a novel FTSJ1-p.A26P missense allele specifically lacks Gm34, but has normal levels of Cm32 and o2yW37. tRNAPhe from the corresponding Saccharomyces cerevisiae trm7-A26P mutant also specifically lacks Gm34, and the reduced Gm34 is not due to weaker Trm734 binding. These results directly link defective 2'-O-methylation of the tRNA anticodon loop to FTSJ1 mutations, suggest that the modification defects cause NSXLID, and may implicate Gm34 of tRNAPhe as the critical modification. These results also underscore the widespread conservation of the circuitry for Trm7-dependent anticodon loop modification of eukaryotic tRNAPhe.

The binding of the transcriptional regulator CTCF to the genome has been implicated in the formation of topologically associated domains (TADs). However, the general mechanisms of folding the genome into TADs are not fully understood. Here we test the effects of deleting a CTCF-rich locus on TAD boundary formation. Using genome-wide chromosome conformation capture (Hi-C), we focus on one TAD boundary on chromosome X harboring ~ 15 CTCF binding sites and located at the long non-coding RNA (lncRNA) locus Firre. Specifically, this TAD boundary is invariant across evolution, tissues, and temporal dynamics of X-chromosome inactivation. We demonstrate that neither the deletion of this locus nor the ectopic insertion of Firre cDNA or its ectopic expression are sufficient to alter TADs in a sex-specific or allele-specific manner. In contrast, Firre’s deletion disrupts the chromatin super-loop formation of the inactive X-chromosome. Collectively, our findings suggest that apart from CTCF binding, additional mechanisms may play roles in establishing TAD boundary formation.

More than half the human and mouse genomes are comprised of repetitive sequences, such as transposable elements (TEs), which have been implicated in many biological processes. In contrast, much less is known about other repeats, such as local repeats that occur in multiple instances within a given locus in the genome but not elsewhere. Here, we systematically characterize local repeats in the genomic locus of the Firre long noncoding RNA (lncRNA). We find a conserved function for the RRD repeat as a ribonucleic nuclear retention signal that is sufficient to retain an otherwise cytoplasmic mRNA in the nucleus. We also identified a repeat, termed R0, that can function as a DNA enhancer element within the intronic sequences of Firre. Collectively, our data suggest that local repeats can have diverse functionalities and molecular modalities in the Firre locus and perhaps more globally in other lncRNAs.