Organization of the Gene Encoding Human Lysosomal β-Galactosidase

Abstract

Human β-galactosidase precursor mRNA is alternatively spliced into an abundant 2.5-kb transcript and a minor 2.0-kb species. These templates direct the synthesis of the classic lysosomal β-D-galactosidase enzyme and of a β-galactosidase-related protein with no enzymatic activity. Mutations in the β-galactosidase gene result in the lysosomal storage disorders G_M1-gangliosidosis and Morquio B syndrome. To analyze the genetic lesions underlying these syndromes we have isolated the human β-galactosidase gene and determined its organization. The gene spans >62.5 kb and contains 16 exons. Promoter activity is located on a 236-bp Pst I fragment which works in a direction-independent manner. A second Pst I fragment of 851 bp located upstream from the first negatively regulates initiation of transcription. The promoter has characteristics of a housekeeping gene with GC-rich stretches and five potential SP1 transcription elements on two strands. We identified multiple cap sites of the mRNA, the major of which maps 53 bp upstream from the translation initiation codon. The portion of the human pre-mRNA undergoing alternative splicing is encoded by exons II–VII. Sequence analysis of equivalent mouse exons showed an identical genomic organization. However, translation of the corresponding differentially spliced murine transcript is interrupted in its reading frame. Thus, the mouse gene cannot encode a β-galactosidase-related protein in a manner similar to the human counterpart. Differential expression of the murine β-galactosidase transcript is observed in different mouse tissues.