Binds and cleaves RNA, particularly in stem-loops. Associates with pre-ribosomal particles in chloroplasts, and participates in chloroplast ribosomal RNA metabolism, probably during the final steps of 23S rRNA maturation. May enhance transcription by the plastid-encoded polymerase and translation in plastid via the stabilization of ribosome assembly intermediates. Required for chloroplast integrity. Involved in the regulation of the circadian system. Involved in the regulation of heteroglycans and monosaccharide mobilization. Required for full expression of genes transcribed by the plastid-encoded RNA polymerase (PEP). Essential for embryo development (PubMed:25161659). Component of a complex made of CSP41A, CSP41B, ribosomes, and the plastid-encoded RNA polymerase. Interacts with CSP41A. Binds DNA when in complex with PRIN2 (PubMed:25161659). Present in stromules (PubMed:16633814). In the cytoplasm, accumulates around chloroplasts (PubMed:21087810). Highly expressed in seedlings, particularly in photosynthetically active organs. Mostly expressed in young and mature leaves, and, to a lower extent, in flowers. Low expression in etiolated seedlings compared to green seedlings. In young seedlings, expressed in hypocotyls and cotyledons. In older seedlings, limited to the outer epidermal cell layer of leaves and petioles (including guard cells). Present in trichomes and hydathodes. In flowers, detected in sepals and siliques. Expressed with a circadian rhythm showing a peak during the end of the day (under long day conditions). Repressed during senescence and upon water stress. Accumulates at wounding sites. Altered expression of both oscillator and output genes. Small and pale plants, with altered chloroplast morphology (anarchic membrane organization) and reduced photosynthetic performance associated with a reduction in CSP41A levels. Altered monosaccharide pattern of heteroglycans. Lethal when associated with CSP41A disruption. Reduced transcript levels of photosynthesis genes. Defects in embryo development. The csp41b-2 prin2-2 double mutant is embryo lethal (PubMed:25161659). Belongs to the NAD(P)-dependent epimerase/dehydratase family.