Genomic organization of the human asparagine transfer RNA genes: localization to the U1 RNA gene and class I pseudogene repeat units.
Previously isolated human DNA clones containing asparagine transfer RNA (tRNAAsn) genes have been used to determine the genomic organization of this multigene family in man. One clone also contained a gene for U1 RNA, and so the organization of the two multigene families could be directly compared. The majority, and perhaps all, of the human tRNAAsn genes map to the same chromosome bands as do the U1 RNA true genes and class I pseudogenes located on the short and long arms, respectively, of chromosome 1. These two gene clusters were independently isolated using a somatic-cell hybrid minipanel, and use of repeat-unit DNA polymorphisms showed that one tRNA gene clone maps to the short-arm gene cluster and the other to the long-arm gene cluster. Electron microscopy of heteroduplexes between these two clones showed duplex formation along the proposed region of overlap between them, indicating that the short- and long-arm gene clusters are structurally related. I suggest that the split into two distinct loci was facilitated by a pericentric chromosome inversion. This would have had the effect of positioning the genes currently on the long arm adjacent to the centromeric heterochromatin, perhaps resulting in a "position effect" on transcription of these genes. Restriction fragments of different sizes were found that were common to a majority of repeat units, depending on the restriction enzyme being used. Pulsed-field electrophoresis revealed that fragments of molecular weight of 180 kb were common to each unit (or multiples of units). These fragments also contained U1 RNA gene sequences. I therefore propose that these two gene families are closely linked on repeat units (or multiples of units) of 180 kb in size, which are probably organized in tandem arrays.