Intron positions are conserved in the 5' end region of myosin heavy-chain genes.

We have determined the 5' end sequence of the rat embryonic skeletal muscle myosin heavy-chain (MHC) gene comprising the first three amino-terminal coding exons. Comparison with the corresponding regions of the rat ventricular alpha and the nematode Caenorhabditis elegans unc-54 MHC genes (Mahdavi, V., Chambers, A.P., and Nadal-Ginard, B. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 2626-2630; Karn, J., Brenner, S., and Barnett, L. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 4253-4257) shows that the degree of amino acid sequence conservation increases from the first to the third exon. Intron positions between these exons are maintained in all three genes studied, whereas size and sequence of corresponding introns are highly divergent. In contrast to the rat MHC genes where the coding region is highly split throughout its entire length, only the 5' end region is frequently interrupted by introns in the nematode gene indicating the potential importance of these introns in gene structure and expression. The occurrence of "preferential" intron positions in the MHC genes suggests the existence of a highly split ancestral MHC gene from which different evolutionary lineages removed and/or added specific sets of introns.