Chloroplast gene expression in Chlamydomonas reinhardi

Abstract

The transcriptional activities of the chloroplast genome of Chlamydomonas reinhardi during the synchronous cell cycle and at various stages of the greening process have been characterized by hybridization of pulse labeled RNA to separated fragments of chloroplast DNA digested with EcoRI endonuclease. The results demonstrate that the EcoRI-chloroplast DNA fragments can be divided into three classes with respect to gene expression: (1) fragments in which gene expression occurs stage specifically, (2) fragments in which gene expression occurs continously, and (3) fragments in which gene expression is absent. A typical example of class 1 DNA fragments are those which contain chloroplast rRNA genes. Several other fragments containing mRNA species are also transcribed stage specifically. The DNA fragment containing the mRNA gene for the large subunit of RuBP-carboxylase appears to belong to class 2. Three EcoRI fragments (19, 20, 22) are not transcribed at all. These fragments were not expressed even when the chloroplast DNA was transcribed in vitro by E. coli RNA polymerase. The buoyant density analysis of DNA fragments shows a distinct base composition hetereogeneity. The three silent fragments have an extremely low bouyant density and are localized in a cluster on the physical map of Rochaix (1978). Four high density regions can be recognized on the map, two of which involve genes for chloroplast rRNAs and one for mRNA for the large RuBP-carboxylase subunit.