The pre-mRNA splicing factor PRPF8 is a crucial component of the U5 snRNP. Using quantitative proteomics, Malinová et al. show that assembly of the U5 snRNP is controlled by the HSP90/R2TP chaperones and that Retinitis pigmentosa–associated mutations in PRPF8 impair PRPF8 quality control and U5 snRNP chaperone-mediated assembly.

DNA replication sites (RS) in synchronized HeLa cells have been studied at the electron microscopic level. Using an improved method for detection following the in vivo incorporation of biotin-16-deoxyuridine triphosphate, discrete RS, or foci are observed throughout the S-phase. In particular, the much larger RS or foci typically observed by fluorescence microscopic approaches in mid- and late-S-phase, are found to be composed of smaller discrete foci that are virtually identical in size to the RS observed in early-S-phase. Pulse-chase experiments demonstrate that the RS of early-S-phase are maintained when chased through S-phase and into the next cell generation. Stereologic analysis demonstrates that the relative number of smaller sized foci present at a given time remains constant from early through mid-S-phase with only a slight decrease in late-S-phase. 3-D reconstruction of serial sections reveals a network-like organization of the RS in early-S-phase and confirms that numerous smaller-sized replication foci comprise the larger RS characteristic of late-S-phase.