title: Differentiation and fiber type-specific activity of a muscle creatine kinase intronic enhancer.
abstract: BACKGROUND
  Hundreds of genes, including muscle creatine kinase (MCK), are differentially expressed in fast- and slow-twitch muscle fibers, but the fiber type-specific regulatory mechanisms are not well understood.
  
  
  RESULTS
  Modulatory region 1 (MR1) is a 1-kb regulatory region within MCK intron 1 that is highly active in terminally differentiating skeletal myocytes in vitro. A MCK small intronic enhancer (MCK-SIE) containing a paired E-box/myocyte enhancer factor 2 (MEF2) regulatory motif resides within MR1. The SIE's transcriptional activity equals that of the extensively characterized 206-bp MCK 5'-enhancer, but the MCK-SIE is flanked by regions that can repress its activity via the individual and combined effects of about 15 different but highly conserved 9- to 24-bp sequences. ChIP and ChIP-Seq analyses indicate that the SIE and the MCK 5'-enhancer are occupied by MyoD, myogenin and MEF2. Many other E-boxes located within or immediately adjacent to intron 1 are not occupied by MyoD or myogenin. Transgenic analysis of a 6.5-kb MCK genomic fragment containing the 5'-enhancer and proximal promoter plus the 3.2-kb intron 1, with and without MR1, indicates that MR1 is critical for MCK expression in slow- and intermediate-twitch muscle fibers (types I and IIa, respectively), but is not required for expression in fast-twitch muscle fibers (types IIb and IId).
  
  
  CONCLUSIONS
  In this study, we discovered that MR1 is critical for MCK expression in slow- and intermediate-twitch muscle fibers and that MR1's positive transcriptional activity depends on a paired E-box MEF2 site motif within a SIE. This is the first study to delineate the DNA controls for MCK expression in different skeletal muscle fiber types.
authors.affiliation: Department of Biochemistry, 1705 NE Pacific St,, University of Washington, Seattle, WA 98195, USA. haus@u.washington.edu.
authors.forename: Phillip Wl
authors.initials: PW
authors.lastname: Tai
authors.forename: Katherine I
authors.initials: KI
authors.lastname: Fisher-Aylor
authors.forename: Charis L
authors.initials: CL
authors.lastname: Himeda
authors.forename: Catherine L
authors.initials: CL
authors.lastname: Smith
authors.forename: Alexandra P
authors.initials: AP
authors.lastname: Mackenzie
authors.forename: Deri L
authors.initials: DL
authors.lastname: Helterline
authors.forename: John C
authors.initials: JC
authors.lastname: Angello
authors.forename: Robert E
authors.initials: RE
authors.lastname: Welikson
authors.forename: Barbara J
authors.initials: BJ
authors.lastname: Wold
authors.forename: Stephen D
authors.initials: SD
authors.lastname: Hauschka