Transcriptional regulation of mammalian genes in vivo. A tale of two templates

The study of mammalian gene expression through the use of transient transfection assays has greatly expanded our knowledge of transcriptional mechanisms. However, transfected promotor constructs do not always serve as appropriate 'stand-ins' for endogenous genes, particularly in cases where chromatin remodeling must take place. The examples described above indicate that a level of caution is advised when studying regulation of various promoters and transcription factor function with the use of transient transfection assays. When possible, function of the corresponding endogenous promoters should be tested to assess the validity of regulatory mechanisms defined on transiently transfected, non-replicating templates. Future studies using template comparison as a tool will undoubtedly identify which transcription factors are critical in initiating essential changes in chromatin structure at endogenous target genes and assist in the elucidation of mechanisms involved in chromatin transitions. As in the MyoD experiments (36), these studies will also allow identification of functional domains in these factors, which are necessary for chromatin remodeling. Functional differences between transiently transfected and stable replicating templates need not be considered artifactual but rather can be exploited to identify and characterize regulatory mechanisms that involve chromatin components. Full understanding of gene expression in vivo will not be achieved until these mechanisms are understood in detail.