Synthesis of the large subunit of ribulose-1,5-bisphosphate carboxylase in anin vitro partially definedE. coli system

The atpB and rbcL promoters in plastid DNAs of a wide dicot range

The plastid atpB-rbcL intergene has been analyzed within a wide range of plants covering the major dicot lineages. New sequences from 13 plant species were determined and aligned with three already-known sequences. The promoters of the rbcL and the atpB genes were localized and analyzed according to published observations in spinach and tobacco. The evolutionary conservation of two atpB promoters, separated by 113-262 nucleotides, is strong support that both are functionally active, and it also allows a discrimination between the previously reported atpB transcripts. Moreover, the radically distinct sequences of the two atpB promoters suggest that they interact with two distinct initiation complexes. The alignment also confirms the much higher conservation of the leader sequence in the rbcL mRNA than in the atpB mRNA among dicots, presuming a function at the posttranscriptional level.

An in vitro transcription termination system to analyze chloroplast promoters: identification of multiple promoters for the spinach atpB gene

Promoters for spinach chloroplast genes were cloned 5' to a strong factor-independent transcription terminator from E. coli. These "minigene" constructions were transcribed in vitro by a transcriptionally active extract of spinach chloroplasts. Transcription of supercoiled DNA templates resulted in synthesis of discretely-sized RNAs that were readily quantifiable. The efficiency of transcription was up to 3.5 RNAs per template. The transcription termination system described in this report was used to identify the primary transcripts for the plastid atpB gene. Four in vivo transcripts for the atpB gene have been previously identified with 5' untranslated leaders of approximately 455, 275, 180 and 100 nucleotides, respectively. In this report we show that the "-455", "-275" and "-180" regions function as chloroplast promoters in vitro. In addition, a fourth promoter was found that yields a primary transcript totally lacking an untranslated leader.

Using bacteria to analyze sequences involved in chloroplast gene expression

The expression of higher plant chloroplast genes in prokaryotic cells has been used to examine organelle sequences involved in promoter recognition by RNA polymerase, and protein translocation through membranes. The similarity in sequence structure between Escherichia coli promoters and the maize chloroplast atpB promoter has been investigated using deletion and single base pair substitution mutants. The atpB mutants were mainly isolated by a selection system in E. coli, and then used as templates for the analysis of transcription using chloroplast RNA polymerase. It was found that both the bacterial and chloroplast RNA polymerases behaved in a similar fashion with the wild-type and mutant promoters, indicating that the sequences involved in promoter recognition share a considerable degree of homology. Signal peptide recognition of pea cytochrome f has also been examined in E. coli. This signal peptide, which is probably responsible for insertion of the protein into the thylakoid membrane, is efficiently recognized in E. coli leading to the inner membrane insertion of petA::lacZ fusion proteins. This process requires the bacterial SecA protein and points to a general similarity in the mechanisms of protein translocation within chloroplasts and bacteria.

One large subunit of ribulose 1,5-bisphosphate carboxylase oxygenase in Medicago, Spinacia and Nicotiana

Isoelectric focusing of subunits of ribulose 1,5-bisphosphate carboxylase oxygenase of Medicago, Spinacia and Nicotiana were investigated, using a rapid isolation technique, without S-carboxymethylation. RuBPC-ase and its subunits were isolated by gel electrophoresis. Isoelectric focusing of RuBPC-ase of M. sativa and M. falcata showed that this enzyme consists of one large subunit (LSU) polypeptide and two or three small subunits (SSU), depending on the genotype. The pl of the LSU's was identical, but the pl of SSU's of the two genotypes was different. Amino acid composition and tryptic peptide maps further supported the concept of a conserved nature of LSU and heterogeneity of SSU polypeptides in Medicago. It was also found that S. oleracea, N. tabacum, N. glutinosa and N. excelsior have a single LSU polypeptide, but they differ in respect of pl values. The SSU polypeptides appeared to be variable. S-carboxymethylation affected the number as well as the pl values of LSU and SSU polypeptides. It is suggested that one LSU polypeptide is probably the general rule in higher plants, rather than the three LSU polypeptides demonstrated by Chen et al. (1977) and Wildman (1979).

An in vitro system to measure gene expression based on dipeptide synthesis

A simplified E. coli in vitro system has been developed to study gene expression based on the synthesis of the first di- or tripeptide of the gene product. Plasmids containing bacterial and chloroplast genes have been used as templates in this system. The expression of the E. coli L10 operon, which is under both transcriptional and translational control, has been investigated in some detail using the dipeptide system. A similar system has been developed, using eukaryotic translation components, to measure the expression of eukaryotic mRNA based on dipeptide formation.

Multiple transcripts for higher plantrbcL andatpB genes and localization of the transcription initiation site of therbcL gene

We have compared therbcL andatpB transcription units from spinach, maize, and pea. In most cases multiple transcripts were found for a given chloroplast gene. The 5' termini of these transcripts were determined by S1 nuclease protection and primer extension analyses. TherbcL transcripts have 5' termini 178-179 and 64 nucleotides (spinach), 300 and 59-63 nucleotides (maize), and 178 and 65 nucleotides (pea) upstream from their respective protein coding regions. TheatpB transcripts have 5' termini (453-454, 272-273, 179, and 99 nucleotides (spinach), 298-302 nucleotides (maize), and 351-355 nucleotides (pea) upstream from their respective protein coding regions. The intergenic distance between therbcL andatpB genes is relatively constant (152 to 157 base pairs) among the three chloroplast genomes. In spinach, maize, and pea, the 80 base pairs surrounding the 5' end of therbcL gene (±40 base pairs) have 85% sequence homology. Similarly, the 60 base pairs preceding theatpB gene have 48% sequence homology. Both genes have '-10' and '-35' regions that resemble the prokaryotic consensus promoter sequence. The larger, but not smaller,rbcL transcripts from spinach and pea can be labeled with alpha-(32)P-GTP by guanylyltransferase. These data suggest that DNA sequences 178-179 (spinach), 300 (maize), and 178 (pea) base pairs before therbcL protein coding regions represent sites of transcription initiation. The sequences 59-65 base pairs before therbcL protein coding regions may correspond to sites of RNA cleavage.

Determination of the translation start site of the large subunit of ribulose-1,5-bisphosphate carboxylase from maize

Sequence studies of the gene for the large subunit of ribulose-1,5-bisphosphate carboxylase from maize indicate the presence of two translation start sites, 18 bp apart. Each site is preceded by a suitable ribosome binding region. By using a simplified E. coli-based in vitro system which measures fromation of the first dipeptide of the gene product, we have determined that only the second methionine initiates translation.