Tissue-specific Expression of the Large ATP Synthase Gene Cluster in Spinach Plastids

Plastids present in different tissues may vary morphologically and functionally, despite the fact that all plastids within the same plant contain identical genomes. This is achieved by regulation of expression of the plastid genome by tissue-specific factors, the mechanisms of which are not fully understood. The proton translocating ATP synthase/ATPase is a multisubunit complex composed of nine subunits, six encoded in the plastid and three in the nucleus. We have investigated the tissue-specific expression of the large ATP synthase gene cluster in spinach (Spinacia oleracea). This gene cluster encodes four of the six plastid-encoded ATP synthase genes. Transcript abundance, transcriptional activity, and transcript stability were investigated relative to gene dosage in root plastids and in stem, leaf, and flower chloroplasts. All three of these factors display significant tissue-specific variation. It was intriguing to discover that, although transcript abundance normalized to gene dosage varies in each tissue, transcript abundance as a proportion of the entire plastid RNA population in each tissue is not significantly different. Thus it appears that in these tissues the variation in transcription and stability of transcripts derived from the large ATP synthase gene cluster balances to yield an equivalent proportion of these transcripts in the plastid RNA population. Expression of this gene cluster in photosynthetic as well as non-photosynthetic tissues may facilitate the plasticity of structure and function which is characteristic of plastids.