Large-conductance Ca(2+)-activated K+ (BK) channels are widespread and functionally heterogeneous. In other classes of K+ channels, functional heterogeneity derives from large gene families, alternative splicing, heterologous subunit composition, and functional modulation. The molecular basis of mammalian BK channel heterogeneity is unknown, since only a single gene (mSlo) has been identified. BK channels in native vascular smooth muscle have an apparent Ca2+ sensitivity approximately 10-fold greater than native brain or skeletal muscle channels, or cloned mSlo channels. Using mSlo as a low-stringency probe, we screened human arterial smooth muscle and genomic libraries extensively in search of genes or splice variants with novel properties. We isolated the human homologue of mSlo, including two novel splice variant forms, but found no other related genes. Electrophysiological characterization of the hSlo clones in Xenopus oocytes and Chinese hamster ovary cells gave BK currents that were not measurably different from mSlo currents. However, coexpression of hSlo with a recently cloned beta-subunit derived from smooth muscle dramatically increased apparent Ca2+ sensitivity. Thus alpha-subunits alone may not determine Ca2+ sensitivity of vascular smooth muscle BK channels. hSlo was mapped to human chromosome 10q23.1, and the genomic structure was analyzed. Immediately after the amino terminal, two unusual regions of trinucleotide repeating sequences are present. The first of these regions encodes polyglycine, and the second encodes polyserine. Both regions of repeated sequence are conserved between the mouse and human genome.

In addition to sustaining an exponentially increasing rate of gene finding (Collins 1995), yeast artificial chromosome/sequence-tagged site (STS/YAC)-based maps (Burke et al. 1987;Olson et al. 1989) have begun to reveal additional features of chromosome structure and dynamics. For example, during the development of maps for subportions of the X chromosome, the existence of a second "pseudoautosomal" region at the Xq terminus of the chromosome was demonstrated (Freije and Schlessinger 1992;Li and Hamer 1995), followed by the discovery that the region shows a unique phenomenon of gene inactivation on both the X and Y homologs (D'Esposito et al. 1996). In another instance, it was shown that a cluster of genes in a delimited segment of XpI 1 escape X inactivation (Miller et al. 1995). As the density of markers across the chromosome has increased beyond the 100-kb resolution goal suggested for the 1Corresponding author. E-MAIL davids@sequencer.wustl.edu; FAX (314) 362-3203."genome initiative," additional features are revealed, as described here.The average inter-STS distance of-75 kb has been achieved by the placement of 2091 STSs on cognate YACs across the 160 Mb of the chromosome. Collectively, the STSs sample -1% of Xspecific sequences. About half of the STSs (962) are made from YAC insert ends (Kere et al. 1992), and another 592 are from randomly derived unique Xchromosomal sequences. However, the STSs also include 97 expressed sequence tags (ESTs) and 190 gene-specific STSs from known genes, as well as 192 dinucleotide and 38 tri-and tetranucleotide repeat markers that detect polymorphism. As a result, the YAC/STS map can be integrated with transcriptional and genetic maps. RESULTS Mapping Strategy and PerformanceWe used a modified "all-walking" form of STS content mapping (Kere et al. 1992) in which STSs were

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A cDNA clone coding for human protein kinase CKI (casein kinase 1) has been isolated and sequenced demonstrating that it corresponds to a homolog of the CKla form found in bovine brain. The derived amino acid sequence of the human CKlcl is identical to the bovine counterpart except that it contains I2 extra amino acids at the carboxyl end. Using this cDNA sequence and PCR amplification, YAC genomic clones that contain this human CKla sequence have been isolated. These YACs have been used for fluorescent in situ hybridization in order to localize the human CKla gene to chromosome 13q13.

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