Many of the plant disease resistance genes that have been isolated encode proteins with a putative nucleotide binding site and leucine-rich repeats (NBS-LRR resistance genes). Oligonucleotide primers based on conserved motifs in and around the NBS of known NBS-LRR resistance proteins were used to amplify sequences from maize genomic DNA by polymerase chain reaction (PCR). Eleven classes of non-cross-hybridizing sequences were obtained that had predicted products with high levels of amino acid identity to NBS-LRR resistance proteins. These maize resistance gene analogs (RGAs) and one RGA clone obtained previously from wheat were used as probes to map 20 restriction fragment length polymorphism (RFLP) loci in maize. Some RFLPs were shown to map to genomic regions containing virus and fungus resistance genes. Perfect cosegregation was observed between RGA loci and the rust resistance loci rp1 and rp3. The RGA probe associated with rp1 also detected deletion events in several rp1 mutants. These data strongly suggest that some of the RGA clones may hybridize to resistance genes.

A cDNA clone of maize Adh1 which contains the entire protein coding region of the gene has been constructed. The protein sequence predicted from the nucleotide sequence is in agreement with limited protein sequencing data for the ADH1 enzyme. An 11.5 kb genomic fragment containing the Adh1 gene has been isolated using the cDNA clone as a probe, and the gene region fully sequenced. The gene is interrupted by 9 introns, their junction sequences fitting the animal gene consensus sequence. Within the gene there is a triplication of a segment (104 bp) spanning an intron-exon junction. Presumptive promoter elements have been identified and are similar in nucleotide sequence and location, relative to the start of transcription, to those of other plant and animal genes. No recognizable poly(A+) addition signal is evident. Comparison of the nucleotide sequences of the cDNA (derived from an Adh1 -F allele) and genomic (derived from an Adh1 -S allele) clones has identified an amino acid difference consistent with the observed difference in electrophoretic mobility of the two enzymes. The maize ADH1 amino acid sequence is 50% homologous to that of horse liver ADH but is only 20% homologous to yeast ADH.

A highly repeated DNA sequence has been isolated from the maize genome as a satellite in actinomycin D/CsCI gradients. By using maize stocks differing in their heterochromatin content we have established that the sequence is a major constituent of one class of heterochromatin, knob heterochromatin, which can occur at 23 locations in the chromosome complement. The repeating unit, of 185 base pairs, has been cloned in plasmid pBR322 and its nucleotide sequence has been determined. The presence of this DNA sequence in knob heterochromatin and its absence from centromeric, nucleolar, and B chromosome heterochromatin parallels the cytogenetic differentiation previously described for these classes of heterochromatin in maize. Because knob heterochromatin has a distinctive cytological appearance and is unique in showing neocentric activity at meiosis, its association with a particular repeated DNA sequence may reflect a functional role for the sequence in the cell cycle.Maize has blocks of heterochromatin in a number of different chromosomal regions. Each centromere is flanked by heterochromatin (centromeric heterochromatin) and there is a large block of heterochromatin at the nucleolus organizer region (NOR heterochromatin). The chromosomes may also have heterochromatin at 23 other sites (knob heterochromatin). The number of knobs varies in different races of maize and can be manipulated in genetic crosses. Heterochromatin is also a major constituent of the supernumerary B chromosomes found in some races of maize.Each class of heterochromatin is distinguishable by its cytological appearance, knob heterochromatin being smooth and sharply delimited in prophase and prometaphase ofboth mitosis and meiosis. Knobs occur at a number oflocations in the normal A chromosomes and there is a small knob in the proximal region of the long arm of the B chromosome. This knob has the same cytological appearance as the A chromosome knobs and it differs from the large diffuse blocks of heterochromatin that occur in the distal half of the long arm of the B chromosome. All knobs have a particular Giemsa staining behavior different from other heterochromatin (1). NOR heterochromatin resembles knobs in its cytological appearance but is sometimes diffuse and vesiculated.The four classes of heterochromatin are also differentiated by their times of replication in the mitotic cycle (2) and by their specific genetic effects. For example, B chromosome heterochromatin enhances recombination frequencies and induces loss of chromosomal segments from knobbed A chromosomes in the second microspore division (3, 4). Some, and possibly all, knobs affect the levels of recombination in particular regions of the chromosome complement. The heterochromatin of the large knob (K10) on abnormal chromosome 10 has the remarkable property ofcausing preferential recovery ofknobbed chromosomes from all knobbed/knobless heterozygotes (5). This departure from normal genetic behavior appears to be a consequence of another known property of K10, the induction of...