Tissue-specific processing of the Surf-5 and Surf-4 mRNAs.

The mouse surfeit locus is an unusually tight cluster of at least six "housekeeping" genes that do not share any sequence homology and whose gene organization may play a role in gene expression. The transcription of each of the five well-characterized genes (Surf-1 to -5) alternates with respect to its neighbor(s) and no more than 159 bp separates any two adjacent genes with the Surf-4 and Surf-2 genes overlapping at their 3' ends by 133 bp. In this work, the expression of the Surf-5 and Surf-4 genes has been examined in various mouse tissues. In addition to the ubiquitously expressed 3.5-kb Surf-5 mRNA, a second alternatively spliced Surf-5 mRNA, Surf-5b, was discovered that was highly expressed in the brain, heart, testis, and skeletal muscle. The alternative splice donor site of the Surf-5b mRNA is similar to splice donor sites found in neuron-specific mRNAs. Surf-5b encodes a unique protein, which, like the ubiquitous Surf-5 protein, has been found to be primarily located in the soluble fraction of the cytoplasm. The expression of the Surf-5b protein was also found to increase in embryonal carcinoma cells differentiated into neuronal cultures. Although the Surf-5 gene is highly conserved through evolution, the presence of the Surf-5b alternative splice may be restricted to higher vertebrates. The Surf-4 gene was ubiquitously expressed in eight different mouse tissues; however, the ratios of the three previously reported Surf-4 mRNAs (two of which are known to derive from different sites of polyadenylation) altered dramatically between tissues. The use of different forms of mRNA processing for regulation of tissue-specific expression of ubiquitously expressed genes is discussed.