Human U1 small nuclear RNA is encoded by approximately 30 gene copies. All of the U1 genes share several kilobases of essentially perfect flanking homology both upstream and downstream from the U1 coding region, but remarkably, for many U1 genes excellent flanking homology extends at least 24 kilobases upstream and 20 kilobases downstream. Class I U1 RNA pseudogenes are abundant in the human genome. These pseudogenes contain a complete but imperfect U1 coding region and possess extensive flanking homology to the true U1 genes. We mapped four class I pseudogenes by in situ hybridization to the long arm of chromosome 1, bands q12-q22, a region distinct from the site on the distal short arm of chromosome 1 to which the U1 genes have been previously mapped (Lund et al., Mol. Cell. Biol. 3:2211-2220, 1983; Naylor et al., Somat. Cell Mol. Genet. 10:307-313, 1984). We confirmed our in situ hybridization results by genomic blotting experiments with somatic cell hybrid lines with translocation products of human chromosome 1. These experiments provide further evidence that class I U1 pseudogenes and the true U1 genes are not interspersed. The results, along with those published elsewhere (Bernstein et al., Mol. Cell. Biol. 5:2159-2171, 1985), suggest that gene amplification may be responsible for the sequence homogeneity of the human U1 gene family.
Partial trisomy of the long arm of human chromosome 1 as demonstrated by in situ hybridization with 5S ribosomal RNA