P1.75 Jan. 4 Extraocular Muscle Myosin and the Origin of Modern Vertebrate Myosins BRIGGS, M.M.; SCHACHAT, F.H.*; Duke Univ. Med. Ctr.; Duke Univ, Med. Ctr. email@example.com
The extraocular muscle (EOM) myosin heavy chain gene (MYH13) is the precursor of "modern" vertebrate striated muscle myosins. EOM myosin is expressed only in EOM and laryngeal muscle of mammals. Phylogenetic analysis implies that MYH13 shared a common ancestor with the progenitor of cardiac myosins, and, through duplication, gave rise to the fast and developmental myosin heavy chain (MYH) cluster in mammals (Schachat and Briggs, 2000). Although it appears to have been the ancestor of the "contemporary" mammalian skeletal muscle myosins, our observations suggest its regulation differs significantly from the other members of the fast and developmental MYHs. Phylogenetic footprinting in human and mouse pointed to likely regulatory sequences in both a proximal promoter region and a region 8 kB upstream of the transcription start site. Studies on transgenic animals and on muscle cells in culture revealed that both the proximal and distal regions are key to the regulation of MYH13 expression. The proximal region lacks most of the myogenic factor binding sites that regulate other fast and developmental MYHs, and it is a negative regulatory element--not a positive one, as in other striated MYHs. The far upstream element is an enhancer that is the essential positive regulatory element required for expression of MYH13. In combination, these two sequences are responsible for the restricted expression of extraocular myosin. This arrangement of positive and negative elements is quite different from that observed in other members of the fast and developmental MYH family. Coupled with phylogenetic analysis, this suggests that the regulation of MYH13 may reflect a more primitive mechanism for MYH transcriptional regulation, which may have been in competition with regulation by myogenic factors.