Abstract
Chloroplasts and other members of the plastid organelle family contain a small genome of bacterial ancestry. Young chloroplasts contain hundreds of genome copies, but the functional significance of this high genome copy number has been unclear. We describe molecular phenotypes associated with mutations in a nuclear gene in maize (Zea mays), white2 (w2), encoding a predicted organellar DNA polymerase. Weak and strong mutant alleles cause a moderate (approximately 5-fold) and severe (approximately 100-fold) decrease in plastid DNA copy number, respectively, as assayed by quantitative PCR and Southern-blot hybridization of leaf DNA. Both alleles condition a decrease in most chloroplast RNAs, with the magnitude of the RNA deficiencies roughly paralleling that of the DNA deficiency. However, some RNAs are more sensitive to a decrease in genome copy number than others. The rpoB messenger RNA (mRNA) exhibited a unique response, accumulating to dramatically elevated levels in response to a moderate reduction in plastid DNA. Subunits of photosynthetic enzyme complexes were reduced more severely than were plastid mRNAs, possibly because of impaired translation resulting from limiting ribosomal RNA, transfer RNA, and ribosomal protein mRNA. These results indicate that chloroplast genome copy number is a limiting factor for the expression of a subset of chloroplast genes in maize. Whereas in Arabidopsis (Arabidopsis thaliana) a pair of orthologous genes function redundantly to catalyze DNA replication in both mitochondria and chloroplasts, the w2 gene is responsible for virtually all chloroplast DNA replication in maize. Mitochondrial DNA copy number was reduced approximately 2-fold in mutants harboring strong w2 alleles, suggesting that w2 also contributes to mitochondrial DNA replication.
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Claims 11
Claim · model-written summary
Weak w2 alleles reduced maize plastid DNA copy number about fivefold.
- Treatment
- Weake0
- Gene
- white2 (w2)e1
- Outcome
- plastid DNA copy numbere0
- Direction
- decreasee0
- Value
- approximately 5-folde0
Zea maysNot reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Weak and strong mutant alleles cause a moderate (approximately 5-fold) and severe (approximately 100-fold) decrease in plastid DNA copy number, respectively”
e1“a nuclear gene in maize (Zea mays), white2 (w2)”
Claim · model-written summary
Strong w2 alleles reduced maize plastid DNA copy number about 100-fold.
- Treatment
- strong mutant allelese0
- Gene
- white2 (w2)e1
- Outcome
- plastid DNA copy numbere0
- Direction
- decreasee0
- Value
- approximately 100-folde0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Weak and strong mutant alleles cause a moderate (approximately 5-fold) and severe (approximately 100-fold) decrease in plastid DNA copy number, respectively”
e1“a nuclear gene in maize (Zea mays), white2 (w2)”
Claim · model-written summary
Both w2 alleles decreased most chloroplast RNAs, with deficiencies roughly paralleling DNA deficiency.
- Treatment
- Both allelese0
- Gene
- white2 (w2)e1
- Outcome
- most chloroplast RNAse0
- Direction
- decreasee0
- Qualifier
- roughly paralleling that of the DNA deficiencye0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Both alleles condition a decrease in most chloroplast RNAs, with the magnitude of the RNA deficiencies roughly paralleling that of the DNA deficiency.”
e1“a nuclear gene in maize (Zea mays), white2 (w2)”
Claim · model-written summary
Some chloroplast RNAs were more sensitive to genome copy number reduction than others.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“some RNAs are more sensitive to a decrease in genome copy number than others”
e1“a nuclear gene in maize (Zea mays), white2 (w2)”
Claim · model-written summary
rpoB mRNA accumulated to dramatically elevated levels after a moderate plastid DNA reduction.
- Treatment
- moderate reduction in plastid DNAe0
- Gene
- rpoBe0
- Outcome
- rpoB messenger RNA (mRNA)e0
- Direction
- elevatede0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“The rpoB messenger RNA (mRNA) exhibited a unique response, accumulating to dramatically elevated levels in response to a moderate reduction in plastid DNA.”
e1“a nuclear gene in maize (Zea mays), white2 (w2)”
Claim · model-written summary
Photosynthetic enzyme subunits were reduced more severely than plastid mRNAs.
- Treatment
- white2 (w2)e1
- Gene
- white2 (w2)e1
- Outcome
- Subunits of photosynthetic enzyme complexese0
- Direction
- reduced more severelye0
- Comparator
- plastid mRNAse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Subunits of photosynthetic enzyme complexes were reduced more severely than were plastid mRNAs”
e1“a nuclear gene in maize (Zea mays), white2 (w2)”
Claim · model-written summary
The authors suggest impaired translation from limiting ribosomal RNA, transfer RNA and ribosomal protein mRNA could explain the enzyme subunit reduction.
- Outcome
- Subunits of photosynthetic enzyme complexese1
- Qualifier
- impaired translation resulting from limiting ribosomal RNA, transfer RNA, and ribosomal protein mRNAe0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“possibly because of impaired translation resulting from limiting ribosomal RNA, transfer RNA, and ribosomal protein mRNA”
e1“Subunits of photosynthetic enzyme complexes were reduced more severely than were plastid mRNAs”
e2“a nuclear gene in maize (Zea mays), white2 (w2)”
Claim · model-written summary
The authors conclude that chloroplast genome copy number limits expression of a subset of chloroplast genes.
- Outcome
- expression of a subset of chloroplast genese0
- Qualifier
- chloroplast genome copy number is a limiting factore0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“chloroplast genome copy number is a limiting factor for the expression of a subset of chloroplast genes in maize”
Claim · model-written summary
w2 is responsible for virtually all chloroplast DNA replication in maize.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the w2 gene is responsible for virtually all chloroplast DNA replication in maize”
Claim · model-written summary
Strong w2 alleles reduced mitochondrial DNA copy number approximately twofold.
- Treatment
- strong w2 allelese0
- Gene
- w2e0
- Outcome
- Mitochondrial DNA copy numbere0
- Direction
- reducede0
- Value
- approximately 2-folde0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Mitochondrial DNA copy number was reduced approximately 2-fold in mutants harboring strong w2 alleles”
e1“a nuclear gene in maize (Zea mays), white2 (w2)”
Claim · model-written summary
The mitochondrial DNA result suggests that w2 contributes to mitochondrial DNA replication.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“suggesting that w2 also contributes to mitochondrial DNA replication”
e1“a nuclear gene in maize (Zea mays), white2 (w2)”