Transcriptome analysis of the Euglena gracilis plastid chromosome

The characterisation of transcript levels of chloroplast genes and their changes under different conditions is an initial step towards understanding chloroplast gene expression and the functional integration of the plastid chromosome into the entire integrated compartmentalised genome of the plant cell. Using RNA from cells of 12 different developmental stages and stress treatments, we have studied the transcript patterns of all 96 genes of the circular plastid chromosome of Euglena gracilis, Pringsheim strain Z, by a macroarray-based approach and Northern analysis of selected genes representing approximately half a dozen operons. The unicellular alga possesses complex, triple-envelope chloroplasts that were acquired by secondary endosymbiosis. (1) Transcripts were detected from all genes, although stationary concentrations varied substantially between individual loci. No obvious economy in the expression pattern with respect to transcription units and genes for complex structures was noted. (2) The chromosome appears to be constitutively expressed under all chosen conditions including stresses such as UV light, temperature, antiplastidial agents, herbicide and heavy metal exposure. (3) The euglenoid organelle transcriptome is qualitatively relatively insensitive to the environment, but exhibited marked overall quantitative changes. The more or less global changes demonstrate that primarily RNA turnover, translational, proteolytic and/or metabolic control regulate organelle gene expression in the alga.