| The expression of the chloroplast genes encoding photosystem II (PSII) subunits requires trans-acting nuclear gene products that act posttranscriptionally in chloroplast RNA processing and maturation (reviewed in Barkan and Goldschmidt-Clermont, 2000), RNA stability (reviewed in Barkan and Goldschmidt-Clermont, 2000), and translation (reviewed in Zerges, 2000). The AC115 nuclear gene has been identified. Its product is essential for the synthesis of the chloroplast-encoded D2 protein of photosystem II. The AC115 gene encodes a 77 amino acid protein that is 8.5 kDa in size and has an isoelectric point of 12.01. Ac115p also contains an N-terminal myristoylation site and three putative phosphorylation sites. Expression of AC115 was studied by Northern blot analysis. This analysis revealed equivalent levels of the AC115 message in both wild type and ac115 strains. In addition, the AC115 mRNA is present in both light- and dark-grown cells. In vitro transport assays revealed that Ac115p is chloroplastic and forms a tight association with the stromal side of thylakoid membranes. Extraction of Ac115p from thylakoid membranes requires treatment with detergent. Localization of the protein to thylakoids does not require thylakoid membrane-associated ribosomes, the PSII complex, or translation of psbD mRNA. It is hypothesized that Ac115p is attached to the thylakoid membrane via a myristic acid and is directly required for D2 expression. Attachment of Ac115p to thylakoids may anchor D2 in the membrane and stabilize the protein allowing for assembly of PSII. |
