A Nuclear Gene in Maize Required for the Translation of the Chloroplast atpB/E mRNA

Abstract

To elucidate mechanisms that regulate chloroplast translation in land plants, we sought nuclear mutations in maize that disrupt the translation of subsets of chloroplast mRNAs. Evidence is presented for a nuclear gene whose function is required for the translation of the chloroplast atpB / E mRNA. A mutation in atp1 results in a failure to accumulate the chloroplast ATP synthase complex due to reduced synthesis of the AtpB subunit. This decrease in AtpB synthesis does not result from a change in atpB mRNA structure or abundance. Instead, the atpB mRNA is associated with abnormally few ribosomes in atp1-1 mutants, indicating that atp1 function is required during translation initiation or early in elongation. Previously, only one nuclear gene that is required for the translation of specific chloroplast mRNAs had been identified in a land plant. Thus, atp1 will be a useful tool for dissecting mechanisms of translational control in chloroplasts. INTRODUCTIONThe chloroplast translation machinery resembles that in eubacteria in many respects (reviewed in Sugiura et al., 1998). Chloroplast ribosomes are similar to bacterial ribosomes in size and antibiotic sensitivities, and the sequences of many chloroplast ribosomal proteins are closely related to those of their prokaryotic ancestors. Despite these similarities, there is increasing evidence that translation mechanisms in chloroplasts differ in several important ways from those in Escherichia coli. For example, chloroplast ribosomes include several proteins that do not have bacterial counterparts (Subramanian, 1993), Shine-Dalgarno sequences play a less prevalent role in chloroplast translation (Sakamoto et al., 1994; Hirose and Sugiura, 1996; Fargo et al., 1998), and the sequence of chloroplast start codons affects translational efficiency but plays only a minor role in positioning the start site (Chen et al., 1995). In addition, there is strong genetic evidence that chloroplast translation is commonly regulated by positively acting regulatory proteins (GoldschmidtClermont, 1998), which is a phenomenon that is rare in E. coli (McCarthy and Gualerzi, 1990).The translation of chloroplast mRNAs is regulated by a variety of factors. For example, light stimulates the translation of both the rbcL and psbA mRNAs (Malnoe et al., 1988;Berry et al., 1990;Staub and Maliga, 1993; Kim et al., 1994) and modulates the rates of both the initiation and elongation steps (Berry et al., 1988(Berry et al., , 1990 Kim et al., 1991; Edhofer et al., 1998). For psbA , this regulation is mediated in part by the 5 Ј untranslated region (UTR) of the mRNA (Staub and Maliga, 1993). Failure to synthesize one component of a photosynthetic complex can influence the translation of other subunits of the same complex: the chloroplast-encoded large subunit of ribulose bisphosphate carboxylase (Rubisco) is translated at reduced rates when synthesis of the nucleusencoded small subunit is disrupted (Khrebtukova and Spreitzer, 1996;Rodermel et al., 1996), and the rate of translation of the petA mRNA is ...

Ribonuclease II preserves chloroplast RNA homeostasis by increasing mRNA decay rates, and cooperates with polynucleotide phosphorylase in 3′ end maturation

“…Finally, nuclear mutants defective in translation of individual mRNAs, do not additionally impact the stability of these mRNAs (e.g. Zerges and Rochaix, 1994; McCormac and Barkan, 1999; Wostrikoff et al. , 2001).…”

The Protein Disulfide Isomerase-like RB60 Is Partitioned between Stroma and Thylakoids in Chlamydomonas reinhardtii Chloroplasts

“…2 and 4 -6). The molecular basis of light-regulated translation in the chloroplast has been shown to be dependent on the function of a growing list of nucleusencoded proteins (7)(8)(9)(10)(11)(12)(13)(14). These factors are thought to mediate translational regulation by interacting with the 5Ј-untranslated region (5Ј-UTR) 1 of chloroplast mRNAs (11,12,(15)(16)(17).…”

Relationship of GUN1 to FUG1 in chloroplast protein homeostasis

“…In WT and gun1‐103 , psaA mRNA is efficiently loaded onto polysomes and hence migrates deep into the Suc gradient (Figure c), as described before (Tadini et al ., ), whereas the low‐molecular‐weight fractions are more enriched in fug1‐3 and fug1‐3 gun1‐103 compared with WT and gun1‐103 plants. This reduced sedimentation rate of the psaA mRNA indicates a reduction in the number of associated ribosomes and implies that translation initiation and/or a step very early in elongation is affected (McCormac and Barkan, ). This result is in line with the function of FUG1 in chloroplast translation initiation and indicates that the defect caused by reduced FUG1 levels is not enhanced by concomitant removal of GUN1 in the fug1‐3 background.…”