Two newly isolated genes from citrus exhibit a different pattern of diurnal expression and light response.

The diurnal expression of two newly isolated genes, ccr and clr, was studied in Duncan grapefruits (Citrus paradisi). The steady-state levels of mRNA corresponding to each of the genes reach their maximum at different time points during the day. The peak of ccr accumulation is at the end of the light period, while accumulation of mRNA corresponding to clr peaks already after 4-6 hours of light. The lowest steady-state levels of mRNA corresponding to both of the genes is at the dark period. ccr is highly homologous to lir1, a gene recently isolated from rice. Comparison of the deduced amino acid sequence of ccr to that deduced from lir1 indicates 49% of identity. The deduced proteins contain two imperfect repeats with two cystein residues that are positionally conserved in each of the repeats. The steady-state level of ccr transcripts continues to oscillate after the plants are transferred to darkness and displays a circadian rhythm similar to that of lir1. On the other hand, the level of clr transcripts is almost undetectable after more than 12 h in the dark. Artificial light supplement in the dark extends the length of time of clr expression during the day. clr does not show significant homology to any of the known circadian or light-regulated genes.