Inhibitory Effect of the Transcription Factor Encoded by the mi Mutant Allele in Cultured Mast Cells of Mice

The mi locus of mice encodes a transcription factor of the basic-helix-loop-helix-leucine zipper protein family (MITF). The MITF encoded by the mutant mi allele ( mi -MITF) deletes 1 of 4 consecutive arginines in the basic domain. The mice of mi/mi genotype express mi -MITF, whereas the mice of tg/tg genotype have a transgene at the 5′ flanking region of the mi gene and do not express any MITF. To investigate the function of mi -MITF in cultured mast cells (CMCs), we took two approaches. First, mRNA obtained from mi/mi CMCs or tg/tg CMCs was subtracted from complementary (c) DNA library of normal (+/+) CMCs, and the (+/+- mi/mi ) and (+/+- tg/tg ) subtraction libraries were obtained. When the number of clones that hybridized more efficiently with +/+ CMC cDNA probe than with mi/mi or tg/tg CMC cDNA probe was compared using Southern analysis, the number was larger in the (+/+- mi/mi ) library than in the (+/+- tg/tg ) library. Second, we compared mRNA expression of six genes between mi/mi and tg/tg CMCs by Northern analysis. The transcription of three genes encoding mouse mast cell proteases was impaired in both mi/mi and tg/tg CMCs. On the other hand, the transcription of three genes encoding c- kit receptor, tryptophan hydroxylase, and granzyme B was markedly reduced in mi/mi CMCs, but the reduction was significantly smaller in tg/tg CMCs. These results indicated the inhibitory effect of mi -MITF on the transactivation of particular genes in CMCs.