Expression of the Large ATP Synthase Gene Cluster in Spinach Plastids during Light-Induced Development

The large ATP synthase gene cluster in spinach (Spinacia oleracea) plastids encodes four of the six chloroplast-encoded ATP synthase subunits. Expression of this cluster was examined to determine its response to light-induced plastid development. Spinach plastid transcripts were isolated from etiolated tissues, etiolated tissues exposed to 24 h of light, young (1-3 cm) leaves, and mature (8-10 cm) leaves. Transcript levels were examined from each developmental stage as a function of either the quantity of total RNA or gene dosage. The relative transcriptional activity of this gene cluster at each of the four developmental stages was also investigated. The stability of these transcripts was deduced by comparing the transcriptional activity with steady-state transcript levels. During the initial 24 h of light-induced development of an etioplast to a chloroplast, transcription decreases in conjunction with increased transcript stability. Transcriptional activity of this cluster per genome then increases between the 24-h and young stages, with a concomitant decrease in the stability of the transcripts. As the young chloroplast matures, the transcripts from this cluster again become markedly more stable, and the transcription of this set of genes declines. Therefore, the regulation of the expression of this cluster is dependent upon a complex interaction between transcriptional and posttranscriptional factors throughout light-induced plastid development.