The Genetics of Corn

Publisher Summary The nucleolus is organized at the nucleolus-organizing region of the chromosomes, which are generally visible as secondary constriction regions in metaphase chromosomes. The chromatin within the constriction region is lost at interphase inside the nucleolar mass. The chromatin is highly extended at this stage. At the ultrastructural level, the nucleolus has at least three components: (1) a granular component consisting mainly of ribonucleoproteins (RNP) granules—pars granulosa, (2) a fibrillar component, consisting of RNP fibrils—pars fibrosa, and (3) chromatin elements. Chromatin elements may be present in three forms: (1) nucleolus-associated chromatin, which most likely does not take part in nucleolus formation; however, the possibility of its association with condensed inactive ribosomal cistrons, at least in some cells, cannot be overruled at present, (2) septalike intranucleolar chromatin, and (3) isolated or dispersed intranucleolar chromatin threads. Intranucleolar chromatin is often associated with the pars fibrosa. Identical components can also be found in isolated nucleoli. Studies on the nucleoli in giant chromosomes indicate that the intranucleolar chromatin in these nucleoli is present as puffs of different sizes. The nucleolar chromatin is not an autonomous structure of the nucleolus but is a continuous structure and part of the nucleolar chromosome.

The nucleolar structure.

Nucleolar dominance is an epigenetic phenomenon that describes nucleolus formation around rRNA genes inherited from only one progenitor of an interspecific hybrid or allopolyploid. The phenomenon is widespread, occurring in plants, insects, amphibians, and mammals, yet its molecular basis remains unclear. We have demonstrated nucleolar dominance in three allotetraploids of the plant genus Brassica. In Brassica napus, accurately initiated pre-rRNA transcripts from one progenitor, Brassica rapa are detected readily, whereas transcripts from the approximately 3000 rRNA genes inherited from the other progenitor, Brassica oleracea, are undetectable. Nuclear run-on confirmed that dominance is controlled at the level of transcription. Growth of B. napus seedlings on 5-aza-2'-deoxycytidine to inhibit cytosine methylation caused the normally silent, under-dominant B. oleracea rRNA genes to become expressed to high levels. The histone deacetylase inhibitors sodium butyrate and trichostatin A also derepressed silent rRNA genes. These results reveal an enforcement mechanism for nucleolar dominance in which DNA methylation and histone modifications combine to regulate rRNA gene loci spanning tens of megabase pairs of DNA.

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Epigenetic silencing of RNA polymerase I transcription: a role for DNA methylation and histone modification in nucleolar dominance

The genes that encode ribosomal RNA exist in two distinct types of chromatin--an 'open' conformation that is permissive to transcription and a 'closed' conformation that is transcriptionally refractive. Recent studies have provided insights into the molecular mechanisms that silence either entire nucleolus organizer regions (NORs) in genetic hybrids or individual rRNA genes within a NOR. An emerging theme from these studies is that epigenetic mechanisms operating at the level of DNA methylation and histone modifications alter the chromatin structure and control the ratio of active and inactive rRNA genes.

Epigenetic silencing of RNA polymerase I transcription

INTRODUCfION ..... , 569 GENE STRUCfURE AND ARRANGEMENT ........ 571 REPLICATION ORIGINS, MEIOTIC PAIRING, AND RECOMBINATION SEQUENCES 573 CLASSIFICATION OF DNA SEQUENCES BY COpy NUMBER 575 THE LENGTH AND CHROMOSOMAL ARRANGEMENT OF SINGLE COpy SEQUENCES 578 REVERSE REPEATS 581 THE STRUCfURE OF REPEATED SEQUENCE ARRAYS 583 THE DISTRIBUTION OF FAMILIES OF REPEATED SEQUENCES IN THE GENOME 586 THE EVOLUTION OF SOME SEQUENCE ORGANIZATION PATTERNS ...... 587 SEQUENCE INTERSPERSION AND CHROMOSOME FUNCTIONS 590 SUMMARY AND CONCLUDING DISCUSSION 591

The molecular characterization and organization of plant chromosomal dna sequences

A maize genetic marker strain (designated as the 2NOR strain) was found to carry a cytologically-visible “duplication” of the nucleolar organizer region (NOR). The 2 NOR condition is a simply inherited cytological marker which is transmitted normally through mega- and microsporogenesis and is associated when homozygous with a nucleolus approximately 64% greater in volume than that of two representative normal inbred lines of maize.—Utilizing DNA · rRNA hybridization techniques and the 2 NOR strain plus two unrelated inbred lines with normal NOR's, the chromosomal site of DNA complementary to rRNA was shown to be localized in the NOR. To our knowledge, this is the first report on the localization of rDNA cistrons in the NOR of plants. An estimate was made of 17,000 rDNA cistrons per diploid nucleus for normal maize.—The 2 NOR strain was shown to possess approximately twice as many rDNA cistrons (34,000) as normal maize. The usefulness of this 2 NOR condition for plant protein improvement programs is being investigated.

S. S. Wang

Papers

The nucleolus organizer region of maize (Zea mays L.): Chromosomal site of DNA complementary to ribosomal RNA