The period gene of the German cockroach and its novel linking power between vertebrate and invertebrate

Circadian clock protein PERIOD (PER) is essential for the endogenous clockworks in diverse lineages within Metazoa, but the protein sequences, the clock protein interactions, and the photic responses are variant and different between vertebrate and invertebrate PER homologs. Here we identified the German cockroach PER homologs and found it could bridge the huge phylogenetic gap and make possible a more precise protein sequence comparison between vertebrate and invertebrate PER homologs. Seven blocks of conserved regions (c1-c7) interspersed within PER proteins were defined, and two new significant homologies were found in the upstream portion of c3 region and in the c7 region, respectively. In addition, we found all dipteran insects PER homologs lack the c7 region and its following amino acid residues. Our results not only reveal the homology and divergence, but also imply the constraint and plasticity of divergent PER proteins during the course of evolution. These findings lay a solid foundation for understanding the general and divergent properties of circadian clockworks in diverse lineages within Metazoa.