Localization and Properties of Transcripts of psbA and rbcL Genes in the Stroma of Spinach Chloroplast

A previous report detailed evidence that mRNA for the 32-kDa protein of PS II and mRNA for the large subunit of ribulosebisphosphate carboxylase/oxygenase (RuBisCO) are present in both the stroma and thylakoid of spinach chloroplast, and that the former mRNA in the stromaand the latter mRNA in the thylakoid fraction are not translated in situ [Minami and Watanabe(1984) Arch. Biochem. Biophys. 235: 562–570.]. Properties of the mRNA in stroma has been examined by hybridization with probes of the respective genes after fractionation of stromal constituents by sucrose density gradient centrifugation. The results indicate that the transcripts of thepsb A gene (mRNA for the 32-kDa protein) are not associated with ribosomes, where as most of transcripts of the rbcL gene (mRNA for the large subunit of RuBisCO) are associated with polysomesin stroma. The extent of accumulation of these transcripts in the non-functioning subchloroplast fractions was determined by quantitative hybridization and was found to be unexpectedly large; close to half of the total population of transcripts of psbA, and more than half ofthat of rbcL were detected in stromal RNA and thylakoid-associated RNA, respectively. These findings are discussed in relation to the translational regulation of gene expression in the chloroplast.