Plastid translation in organello and in vitro during light-induced development in Euglena

Plastids were isolated from dark-grown Euglena gracilis, from cells exposed to light for 3 h, and from normal, light-grown cells. Plastid protein synthesis was then measured either in organello, by incorporation of [35S]Met into isolated plastids, or in vitro, by programming wheat germ, reticulocyte, or Escherichia coli translation systems with RNA isolated from the plastids. Specific and total rates of protein synthesis in organello increased during light-induced development about 100-fold. In contrast, specific mRNA activity of plastid RNA increased no more than 3-fold, and the estimated total mRNA activity of plastids increased less than 10-fold. Protein synthesis in plastids appears therefore to be subject to strong regulation at a level other than that of transcription superimposed on weak transcriptional regulation. The synthesis of the large subunit of ribulose-bisphosphate carboxylase in particular appears to be under translational control. There is also qualitative evidence for regulation. Most plastid mRNAs, as indicated by translation in vitro, electrophoresis, and fluorography of their translation products, increase with light-induced development as a coordinate set. A second or proplastid set can be detected in RNAs from immature plastids, but not from mature chloroplasts. A few mRNAs appear only later in development, corresponding to a delayed set. Most translation products measured in organello also appear to belong to the coordinate set, a few to the proplastid and delayed sets, and a further few to a transient set, which appears only early in development.