Cell type dependent transcription regulation by chick homeodomain proteins.

Five chick homeodomain proteins (CHOXs), CHOX-1.7, -1.1, -1.4, -4.2 and -2.6, had different transcription-regulating activities in a chick cultured cell line, LMH. In particular, CHOX-1.7 highly activated transcription when NP6 was used as the target site whereas CHOX-1.4 did not. This was mainly due to differences in the activation domains since both proteins bound to NP with almost the same affinities in vitro. In LMH cells, they competitively acted on target gene transcription. Moreover, the strength of the CHOX-1.4 activation domain depended on the cell type. These findings suggest that the effect on a target gene is determined by a combination of CHOXs and cell types.