Determinant differences between the rabbit and mouse immunoglobulin kappa enhancers impair the activity of the rabbit enhancer in mouse myeloma cells

OUT, a novel basic helix-loop-helix transcription factor with an Id-like inhibitory activity

Transcription factors belonging to the basic helix-loop-helix (bHLH) family are involved in various cell differentiation processes. We report the isolation and functional characterization of a novel bHLH factor, termed OUT. OUT, structurally related to capsulin/epicardin/Pod-1 and ABF-1/musculin/MyoR, is expressed mainly in the adult mouse reproductive organs, such as the ovary, uterus, and testis, and is barely detectable in tissues of developing embryos. Physical association of OUT with the E protein was predicted from the primary structure of OUT and confirmed by co-immunoprecipitation. However, unlike other bHLH factors, this novel protein failed to bind E-box or N-box DNA sequences and inhibited DNA binding of homo- and heterodimers consisting of E12 and MyoD in gel mobility shift assays. In luciferase assays, OUT inhibited the induction of E-box-dependent transactivation by MyoD-E12 heterodimers. Deletion studies identified the domain responsible for the inhibitory action of OUT in its bHLH and C-terminal regions. Moreover, terminal differentiation of C2C12 myoblasts was inhibited by exogenous introduction of OUT. These inhibitory functions of OUT closely resemble those of the helix-loop-helix inhibitor Id proteins. Based on these findings, we propose that this novel protein functions as a negative regulator of bHLH factors through the formation of a functionally inactive heterodimeric complex.

A novel snail-related transcription factor Smuc regulates basic helix-loop-helix transcription factor activities via specific E-box motifs

Snail family proteins are zinc finger transcriptional regulators first identified in Drosophila which play critical roles in cell fate determination. We identified a novel Snail -related gene from murine skeletalmusclecells designated Smuc. Northern blot analysis showed that Smuc was highly expressed in skeletal muscle and thymus. Smuc contains five putative DNA-binding zinc finger domains in its C-terminal half. In electrophoretic mobility shift assays, recombinant zinc finger domains of Smuc specifically bound to CAGGTG and CACCTG E-box motifs (CANNTG). Because basic helix-loop-helix transcription factors (bHLH) bind to the same E-box sequences, we examined whether Smuc competes with the myogenic bHLH factor MyoD for DNA binding. Smuc inhibited the binding of a MyoD-E12 complex to the CACCTG E-box sequence in a dose-dependent manner and suppressed the transcriptional activity of MyoD-E12. When heterologously targeted to the thymidine kinase promoter as fusion proteins with the GAL4 DNA-binding domain, the non-zinc finger domain of Smuc acted as a transcriptional repressor. Furthermore, overexpression of Smuc in myoblasts repressed transactivation of muscle differentiation marker Troponin T. Thus, Smuc might regulate bHLH transcription factors by zinc finger domains competing for E-box binding, and non-zinc finger repressor domains might also confer transcriptional repression to control differentiation processes.

Comparison of mouse and rabbit Ei kappa enhancers indicates that different elements within the enhancer may mediate activation of transcription and recombination

The intronic Ig kappa-light chain enhancer (Eikappa) has been implicated in regulation of transcription and Vkappa-Jkappa recombination at the kappa locus. To identify sequences within the Eikappa enhancer which are involved in control of recombination, we have made use of the finding that the Eikappa element from the rabbit b9 kappa locus is capable of inducing rearrangement, but not transcription of kappa genes in mouse lymphoid cells. We have therefore compared the binding of murine nuclear proteins to the mouse and rabbit Eikappa elements. DNase I footprinting and gel mobility shift assays indicate that only the kappaB, kappaE1, and kappaE2 sites of the rabbit enhancer are able to interact with murine trans-acting factors. Moreover, although the rabbit kappaB site binds murine NF-kappaB p50/p50 and p50/p65 complexes with high affinity, this site is not capable of mediating transcriptional activation of transient transfection reporter constructs in mouse B lineage cells. These results therefore suggest that, in contrast to the maintenance of kappa enhancer transcription which requires all of the Eikappa sites, only the kappaB, kappaE1, and kappaE2 sites may be necessary for the recombinational activity of the enhancer. Furthermore, NF-kappaB-mediated effects on transcription and recombination appear to involve separate downstream activation pathways.

Evolution of the rabbit immunoglobulin kappa chain genes

We have analyzed the organization and the structure of rabbit kappa chain genes encoding b allotypes in wild rabbits. The kappa 1 gene of the b95 allotype was cloned and its structure determined. The J region is composed of five segments but only J2 appears to be functional and is identical to the J2 segment of the b4 allotype. The J region is highly conserved among the various b allotypes, whereas the constant region exon displays a high level of differences when compared with other allotypes (9%-30% of different amino acids). The b95 J region is closer to that of b4var and the constant region to b5 allotype constant region. Alignment of nucleotide sequences revealed that the constant region exon displays segmental similarities with b4 and bas constant regions. The mosaic structure of b95 allotype gene indicates that complex allotypes of kappa 1 genes may result from genetic exchanges or gene conversion between the different kappa genes.