Cell differentiation by interaction of two HMG‐box proteins: Mat1‐Mc activates M cell‐specific genes in S.pombe by recruiting the ubiquitous transcription factor Ste11 to weak binding sites

The Schizosaccharomyces pombe mfm1 gene is expressed in an M cell‐specific fashion. This regulation requires two HMG‐box proteins: the ubiquitous Ste11 transcription factor and the M cell‐controlling protein Mat1‐Mc. Here we report that the mfm1 promoter contains a single, weak Ste11‐binding site (a so‐called TR‐box) that can confer M‐specificity on a heterologous promoter when present in eight copies. In vitro , both Mat1‐Mc and Ste11 can bind this box with approximately the same affinity. The Mat1‐Mc protein caused a dramatic increase in the DNA‐binding of Ste11 to this box, under conditions where we could not detect Mat1‐Mc in the resulting protein–DNA complex. When we changed a single base in the mfm1 TR‐box, such that it resembled those boxes found in ubiquitously expressed genes, Ste11 binding was enhanced, and in vivo the mfm1 gene also became expressed in P cells where Mat1‐Mc is absent. These findings suggest that M‐specificity results from Mat1‐Mc‐mediated Ste11 binding to weak TR‐boxes. We have also defined a novel motif (termed M‐box), adjacent to the mfm1 TR‐box, to which Mat1‐Mc binds strongly. A DNA fragment containing both the TR‐ and the M‐box allowed the formation of a complex containing both Ste11 and Mat1‐Mc. A single copy of this fragment was sufficient to activate a heterologous promoter in an M‐specific fashion, suggesting that these two boxes act in a synergistic manner.