title: The synthesis and origin of chloroplast low-molecular-weight ribosomal ribonucleic acid in spinach.
abstract: Chloroplasts isolated from young spinach leaves incorporate [3H]uridine into RNA species which co-electrophorese with 5-S rRNA and tRNA, but show very little incorporation into 4.5-S rRNA. Chloroplast 4.5-S rRNA is labelled in vivo after a distinct lag period relative to 5-S rRNA and tRNA. The kinetics of labelling in vivo of chloroplast 5-S rRNA are similar to those of the immediate precursors to the 1.05 x 10(6)-Mr and 0.56 x 10(6)-Mr rRNAs, whereas the kinetics of labelling of the 4.5-S rRNAare similar to those of mature 1.05 x 10(6)-Mr and 0.56 x 10(6)-Mr rRNAs. Chloramphenicol inhibits the labelling of chloroplast 4.5-S rRNA in vivo, and concomitantly inhibits the processing of the immediate precursors to the 1.05 x 10(6)-Mr and 0.56 x 10(6)-Mr rRNAs, but has little effect on the appearance of label in chloroplast 5-S rRNA. DNA/RNA hybridization using 125I-labelled RNAs suggests that chloroplast DNA contains a 2--3-fold excess of 4.5-S and 5-S rRNA genes relative to the high-molecular-weight rRNA genes. Competition hybridization experiments show that the immediate precursor to the 1.05 x 10(6)-Mr rRNA effectively competes with 125I-labelled 4.5-S rRNA for hybridization with chloroplast DNA, and is therefore a likely candidate for  a common precursor to both the 1.05 x 10(6)-Mr and 4.5-S rRNAs.
mesh_terms: D002701:Chloramphenicol / Q000494:pharmacology; D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008970:Molecular Weight; D009693:Nucleic Acid Hybridization; D010944:Plants; D012335:RNA, Ribosomal / Q000096:biosynthesis*; D014158:Transcription, Genetic / Q000187:drug effects; D014529:Uridine
chemical_list: D012335:RNA, Ribosomal; D002701:Chloramphenicol; D014529:Uridine