BASE SUBSTITUTION AT DIFFERENT ALTERNATIVE SPLICE DONOR SITES OF THE TYROSINASE GENE IN MURINE ALBINISM

The c(2j) albino mutation at the mouse tyrosinase locus arose spontane ously in the C57BL/6 inbred strain and causes complete absence of mela nin synthesis, as does the ''classical'' c mutation of long-establishe d albino inbred strains. Sequence analysis of c(2j) cDNA reveals a G - -> T point mutation at nt 291, causing an arginine --> leucine substit ution in codon 77, where the arginine position has been conserved in v ertebrate tyrosinases and tyrosinase-related proteins. While c(2j) dif fers from c, in which there is a G --> C mutation at nt 369 causing a cysteine --> serine substitution, both mutations change the G1 positio n of alternative 5' splice donor sites in exon 1. Both c(2j) and c abo lish the usage of the respective sites for alternative splicing of the tyrosinase pre-mRNA in skin melanocytes. In c(2j), there results an a lmost eightfold increase in activation of the 5' splice site located 7 8 nt downstream, but in c there is no activation of the intact upstrea m splice site. Although the tyrosinase mRNA levels are similar in c(2j ) and wildtype, the protein is virtually absent in c(2j), as in c, pos sibly due to proteolytic degradation. (C) 1996 Academic Press, Inc.