Plant Genome I Conference
Town & Country Conference Center, San Diego, CA, November, 1992.
PG-I: 63pg1
CHROMOPLAST-SPECIFIC GENES AND PROTEINS
Noureddine Hadjeb, Lee Newman, and Carl A. Price, Waksman
Institute, Rutgers University, Piscataway, NJ 0-8855-0759, USA.
Chromoplasts are plastids which contain exceptional amounts
of carotenoids, and are common and highly visible elements of
flowers, fruits, and occasionally roots of higher plants.
Despite their prominence, little is known of the molecular events
that lead to their differentiation. We have identified two
proteins, named ChrA and ChrB, that are specific to chromoplasts
of the sweet pepper, Capsicum annuum. ChrA weighs about 58 kDa,
is an integral membrane protein, and occurs as a
protein-carotenoid complex. ChrB weighs 35kDa and behaves like
a peripheral membrane protein. Neither protein can be detected
immunochemically in organs other than ripening fruits; ChrB
appears early during ripening in all cultivars tested, whereas
ChrA appears late in ripening and then only in red or orange
fruits of cultivars containing the y, gene. ChrB has also been
detected in flowers of Cucumis sativa and possibly in Narcissus
psuedonarcissus and in certain red and orange-red cultivars of
Rosea. Both ChrA and ChrB are encoded by nuclear genes. ChrA
has been isolated from a genomic library from C. annuum prepared
in the vector Lambda FixII, the first exon and upstream sequences
characterized, and the remainder of the gene sequenced but not
mapped with certainty. From the sequence we predict a very
hydrophobic protein, and a transit peptide of 22 amino acids.
The transit peptide shows a near-identity to the transit peptides
of several proteins of the light-harvesting complexes of
Lycopersicon esculentum, which is most surprising. Extending 400
to 800 b upstream from the transcriptional start site of ChrA,
we find an extraordinarily complicated set of "boxes", including
two distinct overlapping sets of novel 42- and 44-bp repeats; we
suspect that these may play a role in the regulation of
expression of this gene. A cDNA containing the N-terminal
sequence of mature ChrB was supplied by R. Schantz and
colleagues. It corresponds to a predicted pre-protein) of
35,220 kDa. Transcripts hybridizing to the cDNA of ChrB are
detectable in unripe fruits, but their abundance increases
enormously early in ripening. ChrA and ChrB are the first truly
chromoplast-specific genes to be isolated. We expect that
analysis of these genes will lead to a better understanding of
the differentiation of plastids into chromoplasts.
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