Sequence of a novel plant ras-related cDNA from Pisum sativum

A clone isolated from a purple podded pea (Pisum sativum L.) cDNA library was shown to contain the complete coding sequence of a polypeptide with considerable homology to various members of the ras superfamily. The ras superfamily are a group of monomeric GTP-binding proteins of 21-25 kDa found in eukaryotic cells. Conserved sequences in the isolated clone include the GTP-binding site, GDP/GTP hydrolysis domain and C-terminal Cys residues involved in membrane attachment. Comparisons of the predicted amino acid sequence with those of other ras proteins show significantly higher homologies (ca. 70%) to two mammalian gene products, those of the BRL-ras oncogene, and the canine rab7 gene, than to any of the plant ras gene products so far identified (< 40% homology). The high percentage of amino acid identity suggests that this cDNA may be the product of a gene, designated Psa-rab, which is the plant counterpart of rab7. Rab/ypt proteins are a subfamily of the ras superfamily thought to be involved in intracellular transport from the endoplasmic reticulum to the Golgi apparatus and in vesicular transport. Northern blot hybridisation analysis of total RNA from green and purple podded pea revealed a mRNA species of approximately the same size as the isolated cDNAs.

The sequence of a gene encoding convicilin from pea (Pisum sativum L.) shows that convicilin differs from vicilin by an insertion near the N-terminus

The sequence of a gene encoding convicilin, a seed storage protein in pea (Pisum sativum L.), is reported. This gene, designated cvcA, is one of a sub-family of two active genes. The transcription start of cvcA was mapped. Convicilin genes are expressed in developing pea seed cotyledons, with maximum levels of the corresponding mRNA species present at 16-18 days after flowering. The gene sequence shows that convicilin is similar to vicilin, but differs by the insertion of a 121-amino-acid sequence near the N-terminus of the protein. This inserted sequence is very hydrophilic and has a high proportion of charged and acidic residues; it is of a similar amino acid composition to the sequences found near the C-terminal of the alpha-subunit in pea legumin genes, but is not directly homologous with them. Comparison of this sequence with the 'inserted' sequence in soya-bean (Glycine max) conglycinin (a homologous vicilin-type protein) suggests that the two insertions were independent events. The 5' flanking sequence of the gene contains several putative regulatory elements, besides a consensus promoter sequence.

Two genes encoding 'minor' legumin polypeptides in pea (Pisum sativum L.). Characterization and complete sequence of the LegJ gene

A genomic clone from pea (Pisum sativum L.) contains all of one gene encoding a 'minor' (B-type) legumin polypeptide, and most of a second very similar gene. The two genes, designated LegJ and LegK, are arranged in tandem, separated by approx. 6 kb. A complete sequence of gene LegJ and its flanking sequences is given, with as much of the sequence of gene LegK as is present on the genomic clone. Hybridization of 3' flanking sequence probes to seed mRNA, and sequence comparisons with cDNA species, suggested that gene LegJ, and probably gene LegK, was expressed. The partial amino acid sequences of 'minor' legumin alpha- and beta-polypeptides were used to confirm the identity of these genes. The transciption start in gene LegJ was mapped. The 5' flanking sequence of gene LegJ contains a sequence conserved in legumin genes from pea and other species, which is likely to have functional significance in control of gene expression. Sequence comparisons with legumin genes and cDNA species from Vicia faba and soya bean show that separation of legumin genes into A- and B-type subfamilies occurred before separation of the Viciae and Glycinae tribes.

Transcription of a legumin gene from pea (Pisum sativum L.) in vitro

The transcriptional activity of the pea legumin gene leg A in the HeLa cell lysate in-vitro transcription system has been assayed. Labelled transcripts were examined by hybridisation to restriction fragments of Leg A, and by glyoxalation followed by agarose gel electrophoresis. Although the legumin gene was not transcribed efficiently, transcripts were shown to be produced from the correct region of Leg A, and analysis by S1 nuclease mapping was used to show that the transcription start was the same as in vivo. The transcripts produced in vitro did not extent over the whole coding sequence of the gene; termination of transcripts occurred in a semi-random fashion. Transcription of templates truncated at their 3' ends (i.e. in coding sequence) showed that discrete transcripts were produced from the start to restriction sites at approx. +520 and +900 bases. Transcription of templates truncated at their 5' ends showed that sequences upstream of-97 bases relative to the transcription start had no appreciable effect on transcription in vitro.

Sequence of a pseudogene in the legumin gene family of pea (Pisum sativum L.)

A second legumin gene, denoted psi Leg D, has been located on the pea genomic clone lambda Leg 1, approx. 1.3 Kbases 3' of Leg A, in the same orientation. The complete sequence of psi Leg D shows that it is a pseudogene, having two stop codons near the 5' end of its predicted coding sequence, as well as deletions and frame shift errors when compared to Leg A. No transcripts from this gene could be detected in developing pea seeds. Leg A and psi Leg D are homologous over their coding sequences, and partially homologous in the intron sequences and the immediate 5' flanking sequences. Other flanking sequences of the two genes show no significant homology, apart from the presence of polyadenylation signals 3' to both coding sequences. The introns in the two genes occur in corresponding positions in the sequences, but a deletion in psi Leg D affects the 3' boundary of IVS-2. Hybridisation of psi Leg D to pea genomic DNA suggests that it does not represent a hitherto undetected sub-family of legumin genes.

Control of storage-protein synthesis during seed development in pea (Pisum sativum L.)

The tissue-specific syntheses of seed storage proteins in the cotyledons of developing pea (Pisum sativum L.) seeds have been demonstrated by estimates of their qualitative and quantitative accumulation by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and rocket immunoelectrophoresis respectively. Vicilin-fraction proteins initially accumulated faster than legumin, but whereas legumin was accumulated throughout development, different components of the vicilin fraction had their predominant periods of synthesis at different stages of development. The translation products in vitro of polysomes isolated from cotyledons at different stages of development reflected the synthesis in vivo of storage-protein polypeptides at corresponding times. The levels of storage-protein mRNA species during development were estimated by 'Northern' hybridization using cloned complementary-DNA probes. This technique showed that the levels of legumin and vicilin (47000-Mr precursors) mRNA species increased and decreased in agreement with estimated rates of synthesis of the respective polypeptides. The relative amounts of these messages, estimated by kinetic hybridization were also consistent. Legumin mRNA was present in leaf poly(A)+ RNA at less than one-thousandth of the level in cotyledon poly(A)+ (polyadenylated) RNA, demonstrating tissue-specific expression. Evidence is presented that storage-protein mRNA species are relatively long-lived, and it is suggested that storage-protein synthesis is regulated primarily at the transcriptional level.