Primary and domain structure of MADS-domain proteins. (a) Sequence logo of the MADS domain based on 6,668 sequences belonging to the MADS superfamily (accession number cl00109) as defined by the Conserved Domains Database at the National Center for Biotechnology Information . Sequences were aligned using hmmalign of the HMMer package  and the logo was created using WebLogo . The logo displays the frequencies of amino acids at each position of the MADS domain, as the relative heights of letters, along with the degree of conservation as the total height of a stack of letters, measured in bits of information. The conserved motif KR[K/R]X4KK, which serves as part of the nuclear localization signal, is indicated by a black line. (b) Domain structure of MADS domain proteins in plants. Type I proteins do not have distinct conserved domains other than the SRF-like MADS domain whereas the MEF2-like MADS domain of type II (MIKC-type) proteins of plants is followed by the intervening (I), the keratin-like (K) and the C-terminal (C) domains. Orange indicates a role in DNA binding, blue denotes a role in protein-protein interaction, and purple indicates a role in transactivation. The three subdomains of the K domain, K1, K2 and K3, are indicated by black lines.