Dispensable chromosomes in addition to the normal complement in diverse taxa are called B chromosomes. The maize B chromosome is discernible in mitotic chromosome spreads as a small compact chromosome composed mainly of heterochromatin. Although much of this chromosome consists of repetitive elements common to the A chromosomes, several sequences specific to the B chromosome have been identified. In the work described here we used the sequence from a B-specific RAPD (random amplification of polymorphic DNA) marker, pBGBM18.2, to isolate another DNA element, StarkB, present in many copies on the B chromosome. StarkB was mapped to the third and fourth blocks of distal heterochromatin using translocation breakpoints and fluorescent in-situ hybridization (FISH). Sequence analysis revealed that StarkB is composed of fragments from the A genome as well as B-specific sequences. The StarkB element is much larger than the other B-specific elements and is not present in large tandem arrays. Different copies of StarkB varied by small insertions, deletions, and duplications as well as single-nucleotide polymorphisms. Reverse transcriptase PCR showed that portions of the StarkB element are expressed. Using the LTR divergence of retroelements interrupting the B-specific sequences, the minimum age of the StarkB repeat array and, by inference, of the B chromosome, was estimated to be 2 million years.
The supernumerary B chromosome has no apparent effects on plant growth, and its molecular makeup is difficult to unravel, due to its high homology to the normal complement, which prevents conventional cloning. This difficulty was overcome previously by microdissecting the B chromosome under the microscope to result in 19 B clones, one of which is B specific and highly repetitive, dispersing over one-third of the B long arm and most regions of the centromeric knob. To gain insights into the molecular structure of the B chromosome, this sequence was used to screen a genomic library constructed from W22 carrying 16 B's. Five clones (Ͼ10 kb each) were isolated, and all were repetitive, showing homology with A chromosomes in Southern and FISH analyses. Two of them were further characterized and sequenced. Each is composed of several restriction fragments with variable degrees of repetitiveness. Some of these are B specific and others have variable degrees of homology with the A chromosomes. The order of each characteristic group is not contiguous; they intersperse within those of other groups. Sequence analysis reveals that their sequences 62ف( kb) have no homology with any published gene other than sequences of transposable elements (retrotransposons and MITEs) and the B as well as the A centromeres. We uncovered a 1.6-kb CL-repeat sequence, seven units of which were present in the two clones in defective forms. Those repeats mostly arrange in tandem array in the B chromosome. Moreover, we detected transposition of a retrotransposon and a MITE element involved in the genesis of these two sequences.T HE maize B chromosome was originally identified pBPC51, is B specific and was mapped-by a series of systematic deletions (hypoploid of B-10L translocaby Kuwada (1915) and has been a subject of extensive cytogenetic studies ever since. Yet, little is known tions)-to the distal heterochromatic region of the B long arm, a map position that was substantiated by FISH about its molecular structure and organization because of difficulty in cloning of B sequences, owing to high analysis. This sequence provides a unique opportunity for isolation of large B sequences by conventional protohomology between the B and the standard complement (A chromosomes), which prevents access of conventional cols.In this article, we took advantage of the unusual propapproaches. This hindrance was overcome by Cheng and Lin (2003). They used a micromanipulator to diserties of this clone-its B-specific and repetitive nature-to screen large fragments of the B chromosome sect pachytene B chromosomes out of a slide under a microscope, and the B fragments were amplified by from a -library constructed from genomic DNA carrying 16 B chromosomes. Five clones were obtained, degenerate oligonucleotide-primed PCR. Cloning of the resulting products resulted in 19 B clones, which hybridtwo of which were further sequenced and characterized. We found retrotransposons, a miniature inverted-repeat ized with genomic DNA in a B-dosage-dependent manner and wi...
In conclusion, the present study demonstrated that the quality and integrity of HSA molecule in dialysis patients were subtly altered and impaired its biological properties. Oxidative alterations of this major plasma protein might adversely affect its vasculoprotective effects in dialysis patients.
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