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 ...

Throughout most of its growth and development, Amaranthus tricolor produces fully green leaves. However, near the onset of flowering, unique leaves emerge that consist of three distinct color regions: green apices, yellow middle regions, and red basal regions. The green apices are identical to fully green leaves in terms of pigment composition, photosynthetic function, and C, gene expression. The yellow and red regions possess greatly reduced levels of chlorophyll and they lack photosynthetic activity. The absence of photosynthetic capacity in the nongreen leaf regions was associated with three distinct alterations in C, gene expression. First, there was a reduction in the translation of C, polypeptides, and in the yellow regions synthesis of the ribulose-l,5-bisphosphate carboxylase small subunit occurred in the absence of large subunit synthesis. Second, there was a reduction in the relative transcription rates of two plastid-encoded photosynthetic genes, rbcL and psbA. Third, there was a loss of bundle-sheath cell-specific accumulation of the r b d and RbcS mRNAs (but not the polypeptides, which remained bundle-sheath-specific). This study indicates that alterations in photosynthetic activity or developmental processes responsible for the loss of activity can influence C, gene expression at multiple regulatory levels.

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 ...

No abstract

In amaranth, a C 4 dicotyledonous plant, the plastid rbcL gene (encoding the large subunit of ribulose-1,5-bisphosphate carboxylase) is regulated post-transcriptionally during many developmental processes, including light-mediated development. To identify posttranscriptional regulators of rbcL expression, three types of analyses (polysome heel printing, gel retardation, and UV cross-linking) were utilized. These approaches revealed that multiple proteins interact with 5 regions of rbcL mRNA in light-grown, but not etiolated, amaranth plants. Light-associated binding of a 47-kDa protein (p47), observed by UV cross-linking, was highly specific for the rbcL 5 RNA. Binding of p47 occurred only with RNAs corresponding to mature processed rbcL transcripts (5-untranslated region (UTR) terminating at ؊66); transcripts with longer 5-UTRs did not associate with p47 in vitro. Variations in the length of the rbcL 5-UTR were found to occur in vivo, and these different 5 termini may prevent or enhance light-associated p47 binding, possibly affecting rbcL expression as well. p47 binding correlates with light-dependent rbcL polysome association of the fully processed transcripts in photosynthetic leaves and cotyledons but not with cell-specific rbcL mRNA accumulation in bundle sheath and mesophyll chloroplasts.