EXPRESSION OF MYOSIN HEAVY-CHAIN TRANSCRIPTS IN NORMAL AND CARDIAC MUTANT MEXICAN AXOLOTLS

In this study, we have cloned a 1.0 kb myosin heavy chain (MHC) cDNA b y screening an axolotl heart cDNA library with the monoclonal antibody MF20 against a light meromyosin (LMM) region of MHC. The nucleotide s equence analysis shows 85-86% homology at the amino acid and 78-81% ho mology at the nucleic acid level with MHC from other vertebrates. Phyl ogenetic analyses suggest that axolotl beta-MHC forms a cluster with t he myosin II group of vertebrate striated muscles. Within the myosin I I cluster, axolotl beta-MHC forms a distinct subclade from avian MHC a nd is instead closer to mammalian MHC. RT-PCR analyses show that trans cripts of beta-MHC are present at stage 2 and the onset Of the MHC gen e expression is at stage 8-10 (gastrulation). Expression increases wit h embryonic development and reaches a maximum at stage 20. Beyond stag e 35 the heart-beat initiation stage, the expression level of beta-MHC is higher in cardiac muscle than in skeletal muscle. We could not det ect significant differences in the levels of expression of MHC transcr ipts in normal and cardiac lethal mutant ((c) under bar/(c) under bar) axolotls (Ambystoma mexicanum).