The TCP domain: a motif found in proteins regulating plant growth and development

The cycloidea (cyc) and teosinte branched 1 (tb1) genes code for structural ly related proteins implicated in the evolution of key morphological traits . However, the biochemical function of CYC and TB1 proteins remains to be d emonstrated. To address this problem, we have analysed the predicted second ary structure of regions conserved between CYC and TB1, and looked for rela ted proteins of known function. One of the conserved regions is predicted t o form a non-canonical basic-Helix-Loop-Helix (bHLH) structure. This domain is also found in two rice DNA-binding proteins, PCF1 and PCF2, where it ha s been shown to be involved in DNA-binding and dimerization. This indicates that the conserved domain moist probably defines a new family of transcrip tion factors, which we have termed the TCP family after its first character ised members ((T) under bar B1, (C) under bar YC and (P) under bar CFs). Ot her plant proteins of unknown function also belong to this family. We have studied two of these in Arabidopsis and have shown that they are expressed in rapidly growing floral primordia. This, together with the proposed invol vement of eye and tb1 in influencing meristem growth, suggests that many me mbers of the TCP family may affect cell division. Some of these genes may h ave been recruited during plant evolution to generate new morphological tra its.