IDENTIFICATION IN A FISH SPECIES OF 2 ID (INHIBITOR OF DNA BINDING DIFFERENTIATION)-RELATED HELIX-LOOP-HELIX FACTORS EXPRESSED IN THE SLOW OXIDATIVE MUSCLE-FIBERS/

Helix-loop-helix (HLH) proteins related to the inhibitor of DNA bindin g/differentiation (Id) serve as general antagonists of cell differenti ation. They lack a basic DNA-binding domain and are thought to functio n in a dominant negative manner by sequestering basic HLH (bHLH) trans cription factors that are involved in cell determination and different iation. Four Id-encoding genes have been shown in mammals, they have a distinct pattern of expression suggesting different functions for eac h member in different cell lineage. In this study we describe the iden tification and cloning of two trout cDNAs which encode helix-loop-heli x proteins showing a high degree of similarity with mammalian Id famil y members. One cDNA encodes a trout putative Id1 protein (TId1) that i s 63% identical to the human Id1 protein over the entire length and 78 % identical within the HLH region. The other cDNA encodes a trout puta tive Id2 protein (TId2) that shows 82% identity to the human Id2 prote in and only one change that is conservative over the HLH region. In th e 3' untranslated region. TId2 mRNA exhibits 16 nucleotides upstream f rom the AATAAA site, a palindromic sequence similar to the cytoplasmic polyadenylation element (CPE) which is also present in Id2 and Id3 mR NAs from mammals and in XIdx/XIdI mRNA from Xenopus. In the fish. TId1 and TId2 are expressed in a tissue-specific manner, with slightly dif ferent patterns. During myogenesis. TId1 and TId2 are highly expressed in the myotomal musculature of fish embryos and of early alevins but are down-regulated in that of late alevins. In muscle from juveniles a nd adults, TId1 and TId2 transcripts are abundant in the slow oxidativ e fibers while they are absent in the fast glycolytic fibers. This exp ression pattern suggests that Id genes play a role in the regulation o f muscle fiber phenotype in addition to controlling early myogenesis. On the whole, the identification of two HLH-Id encoding genes in a maj or taxonomic group like teleosts, suggests an early divergence of Id g enes in vertebrate evolution. The observation that Id transcripts are present selectively in the slow muscle reveals that their expression i s more complicated than previously appreciated.