High level desmin expression depends on a muscle-specific enhancer.

Expression of the desmin gene is increased several-fold during myogenesis. To identify the DNA sequences involved in this regulation, a 16-kilobase fragment containing a 4.5-kilobase 5'-flanking region and the complete sequence of the human desmin gene has been isolated. The transcription initiation sites were determined by S1 nuclease mapping and primer extension analysis. A series of constructs have been made by fusing different lengths of human desmin 5'-flanking fragments to the bacterial chloramphenicol acetyl transferase (CAT) gene, these constructs were introduced either into mouse myogenic cells or into nonmyogenic cells. Comparison of the transient expression of CAT between myogenic and nonmyogenic cells demonstrated that the sequence including 228 base pairs upstream of the transcription initiation site is sufficient to confer low level, muscle-specific expression of the desmin gene. A negative region was located between -693 and -228 base pairs relative to the desmin mRNA start site. High level expression of the gene depends on a 280-base pair muscle-specific enhancer located between -693 and -973 base pairs upstream of the transcription initiation site. This enhancer can activate either the desmin promoter or heterologous promoters about 20-70-fold in myogenic cells. The enhancer, which also acts in myoblast, is discussed in the context of early expression of the desmin during skeletal myogenesis in the mammalian embryo.