Restriction endonucleases cut and partially removed DNA throughout fixed air-dried human metaphase chromosomes. Some enzymes produced a G-banding pattern; some revealed the presence of multiple chromosome-specific classes of highly repetitive DNA in C-band heterochromatin. Enzymes that produced the informative C-band patterns had recognition sequences that were four or five, but not six, base pairs long and did not contain a cytosine-guanine doublet. In both rat and human chromosomes, regions containing amplified ribosomal RNA genes were specifically removed by the restriction endonuclease Msp I.
A region of the TLb locus encompassing T11 to T13 contains retroviral sequences TLev1 and TLev2. As part of a study to determine whether the retroviral elements are involved in the expression of TL genes, the genomic organization of this region was reexamined in greater detail. A result of these investigations is the extension of the H-2 TLb molecular map. Two additional TL genes have been isolated from C57BL/6 mice, T14 and T15. The genomic organization of T9 through T15 is presented. The nucleotide sequence has been determined for exons 4, 5, and 6 of T13. As a result of a C to T conversion, a termination codon is introduced into exon 4, indicating that T13 either encodes a secreted protein or is a pseudogene. T13 was found to be more homologous to the H-2 genes outside the TL region. T14 has been physically disrupted by the integration of TLev1, and the H-2 sequences appear to have diverged greatly. The relationship of the TL regions of the b and c haplotypes has been investigated using numerous low copy probes. The genome of BALB/c (TLc) is shown to lack a counterpart of the T13-T15b region. Homologous regions exist in the two haplotypes; yet considerable polymorphism is observed. TLb mice do not express TLa on the cell surface of normal thymocytes while TLc mice do; TLa expression is activated in many TLb leukemias. The diversity seen in the T13-T15 region may provide insights into the phenotypic expression or regulatory mechanisms of TL expression in these two haplotypes.
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