Knock-out of the genes coding for the Rieske protein and the ATP-synthase delta-subunit of Arabidopsis. Effects on photosynthesis, thylakoid protein composition, and nuclear chloroplast gene expression.

In Arabidopsis, the nuclear genes PetC and AtpD code for the Rieske protein of the cytochrome b(6)/f (cyt b(6)/f) complex and the delta-subunit of the chloroplast ATP synthase (cpATPase), respectively. Knock-out alleles for each of these loci have been identified. Greenhouse-grown petc-2 and atpd-1 mutants are seedling lethal, whereas heterotrophically propagated plants display a high-chlorophyll (Chl)-fluorescence phenotype, indicating that the products of PetC and AtpD are essential for photosynthesis. Additional effects of the mutations in axenic culture include altered leaf coloration and increased photosensitivity. Lack of the Rieske protein affects the stability of cyt b(6)/f and influences the level of other thylakoid proteins, particularly those of photosystem II. In petc-2, linear electron flow is blocked, leading to an altered redox state of both the primary quinone acceptor Q(A) in photosystem II and the reaction center Chl P700 in photosystem I. Absence of cpATPase-delta destabilizes the entire cpATPase complex, whereas residual accumulation of cyt b(6)/f and of the photosystems still allows linear electron flow. In atpd-1, the increase in non-photochemical quenching of Chl fluorescence and a higher de-epoxidation state of xanthophyll cycle pigments under low light is compatible with a slower dissipation of the transthylakoid proton gradient. Further and clear differences between the two mutations are evident when mRNA expression profiles of nucleus-encoded chloroplast proteins are considered, suggesting that the physiological states conditioned by the two mutations trigger different modes of plastid signaling and nuclear response.

The metal ion transporter IRT1 is necessary for iron homeostasis and efficient photosynthesis in Arabidopsis thaliana.

The mutants irt1-1 and irt1-2 of Arabidopsis thaliana were identified among a collection of T-DNA-tagged lines on the basis of a decrease in the effective quantum yield of photosystem II. The mutations responsible interfere with expression of IRT1, a nuclear gene that encodes the metal ion transporter IRT1. In irt1 mutants, photosensitivity and chlorophyll fluorescence parameters, as well as abundance and composition of the photosynthetic apparatus, are significantly altered. Additional effects of the mutation under greenhouse conditions, including chlorosis and a drastic reduction in growth rate and fertility, are compatible with a deficiency in iron transport. Propagation of irt1 plants on media supplemented with additional quantities of iron salts restores almost all aspects of wild-type behaviour. The irt2-1 mutant, which carries an En insertion in the highly homologous IRT2 gene of Arabidopsis thaliana, was identified by reverse genetics and shows no symptoms of iron deficiency. This, together with the finding that irt1-1 can be complemented by 35S::IRT1 but not by 35S::IRT2, demonstrates that, although the products of the two genes are closely related, only AtIRT1 is required for iron homeostasis under physiological conditions.

ATF/CREB sites present in sub-telomeric regions of Saccharomyces cerevisiae chromosomes are part of promoters and act as UAS/URS of highly conserved COS genes.

A highly conserved 48 bp DNA element was identified present at 26 chromosome ends of Saccharomyces cerevisiae. Each element harbours an ideal or a mutated ATF/CREB site, which is a well-known target sequence for bZip transcription factors. In all cases, the sub-telomeric ATF/CREB site element (SACE) is a direct extension of the respective sub-telomeric coreX element. Eight SACEs are part of very long quasi-identical regions of several kilobases, including a sub-telomeric COS open reading frame. Three of these eight SACEs harbour an ideal ATF/CREB site, four a triple-exchange variant (5'-ATGGTATCAT-3'; GTA variant), and one a single exchange variant with a C to G exchange at the left side of the center of symmetry. We analyzed the function of the SACE of the left arm of chromosome VIII in vivo and found its ATF/CREB site to act as UAS/URS of the COS8 promoter, effected by the yeast bZip proteins Sko1p, Aca1p, and Aca2p. Cos8 protein was found in proximity to the nuclear membrane, where it accumulated, especially during cell division. When the ATF/CREB site of the COS8 promoter was exchanged with the GTA variant, the regulation was changed. COS8 was then regulated by Hac1p, a bZip protein known to be involved in the unfolded protein response of S. cerevisiae, indicating, for the first time, a possible functional category for the Cos proteins of S. cerevisiae.