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tk Enzyme expression in differentiating muscle cells is regulated through an internal segment of the cellular tk gene
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.
Insights
Skeletal muscle cells regulate thymidine kinase (tk) enzyme expression during differentiation. The cellular tk gene, but not the herpesvirus tk gene, shows proper regulation, indicating an intragenic regulatory element.
Area of Science:
- Molecular Biology
- Cellular Differentiation
- Gene Regulation
Background:
- Thymidine kinase (tk) enzyme expression decreases during skeletal muscle cell differentiation.
- This regulation is linked to a reduction in cellular tk messenger RNA (mRNA).
- Understanding the genetic basis of this regulation is crucial for muscle development research.
Purpose of the Study:
- To investigate the genetic elements responsible for regulating thymidine kinase (tk) enzyme expression during skeletal muscle terminal differentiation.
- To determine if the cellular tk gene or the herpesvirus tk gene contains the necessary elements for regulated expression.
Main Methods:
- Transfection of tk-deficient mouse myoblasts with chicken cellular tk gene and herpesvirus tk gene.
- Analysis of tk enzyme activity and tk mRNA levels in differentiated and undifferentiated myoblasts.
- Gene manipulation studies involving promoter and 3' terminus replacement.
Main Results:
- Myoblasts expressing the cellular tk gene demonstrated regulated tk enzyme activity upon differentiation.
- Myoblasts expressing the herpesvirus tk gene showed continuous tk enzyme activity post-differentiation.
- Replacing the cellular tk promoter or 3' terminus with viral counterparts did not alter regulated expression, suggesting intragenic control.
Conclusions:
- The cellular tk gene possesses an intragenic element that dictates its regulated expression during muscle differentiation.
- The herpesvirus tk gene lacks this specific regulatory element.
- This finding highlights the importance of intragenic sequences in controlling gene expression during terminal differentiation.