A knob-associated tandem repeat in maize capable of forming fold-back DNA segments: are chromosome knobs megatransposons?
A class of tandemly repeated DNA sequences (TR-1) of 350-bp unit length was isolated from the knob DNA of chromosome 9 of Zea mays L. Comparative fluorescence in situ hybridization revealed that TR-1 elements are also present in cytologically detectable knobs on other maize chromosomes in different proportions relative to the previously described 180-bp repeats. At least one knob on chromosome 4 is composed predominantly of the TR-1 repeat. In addition, several small clusters of the TR-1 and 180-bp repeats have been found in different chromosomes, some not located in obvious knob heterochromatin. Variation in restriction fragment fingerprints and copy number of the TR-1 elements was found among maize lines and among maize chromosomes. TR-1 tandem arrays up to 70 kilobases in length can be interspersed with stretches of 180-bp tandem repeat arrays. DNA sequence analysis and restriction mapping of one particular stretch of tandemly arranged TR-1 units indicate that these elements may be organized in the form of fold-back DNA segments. The TR-1 repeat shares two short segments of homology with the 180-bp repeat. The longest of these segments (31 bp; 64% identity) corresponds to the conserved region among 180-bp repeats. The polymorphism and complex structure of knob DNA suggest that, similar to the fold-back DNA-containing giant transposons in Drosophila, maize knob DNA may have some properties of transposable elements.
Ananiev
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EV
Department of Agronomy and Plant Genetics and Plant Molecular Genetics Institute, University of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108, USA.
Phillips
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RL
Rines
H W
HW
D001483:Base Sequence; D002874:Chromosome Mapping; D004251:DNA Transposable Elements; D018744:DNA, Plant / Q000235:genetics*; D017404:In Situ Hybridization, Fluorescence; D008969:Molecular Sequence Data; D012091:Repetitive Sequences, Nucleic Acid; D015183:Restriction Mapping; D003313:Zea mays / Q000235:genetics*
D004251:DNA Transposable Elements; D018744:DNA, Plant