tk Enzyme expression in differentiating muscle cells is regulated through an internal segment of the cellular tk gene.

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Abstract
Thymidine kinase (tk) enzyme expression is shut down when cultured skeletal muscle cells terminally differentiate. This regulation is mediated by a rapid and specific decline in the abundance of cellular tk mRNA. tk-deficient mouse myoblasts were transformed to the tk-positive phenotype by using both the cellular tk gene of the chicken and the herpesvirus tk gene. Myoblasts transformed with the cellular tk gene effectively regulate tk enzyme activity upon terminal differentiation. Conversely, myoblasts transformed with the herpesvirus tk gene continue to express tk enzyme activity in postreplicative muscle cells. A regulated pattern of expression is retained when the promoter of the cellular tk gene is replaced by the promoter of the herpesvirus tk gene. Moreover, the cellular tk gene is appropriately regulated during terminal muscle differentiation when its 3' terminus is removed and replaced by the terminus of the viral tk gene. Thus, the element of the cellular tk gene sufficient to specify its regulation is entirely intragenic.