A PERIOD-EXTENDER GENE, PEX, THAT EXTENDS THE PERIOD OF THE CIRCADIANCLOCK IN THE CYANOBACTERIUM SYNECHOCOCCUS SP. STRAIN PCC-7942

We cloned the pS1K1 plasmid in the process of apparently ''complementi ng'' a circadian clock mutant of cyanobacterium Synechococcus sp. stra in PCC 7942, SP22, which has a 22-h period (T. Kondo, N. F. Tsinoremas , S. S. Golden, C. H. Johnson, S. Kutsuna, and M. Ishiura, Science 266 :1233-1236, 1994), Sequence analysis revealed that SP22 did not have a mutation in the genomic DNA segment carried on pS1K1, and the sp22 mu tation was later found in a recently cloned new clock gene, kaiC. Ther efore, the period-extender gene pex that was carried on pS1K1 was a su ppressor gene for the sp22 mutation. The par gene encoded a protein of 148 amino acid residues. No meaningful homologs were found in DNA or protein databases including the Synechocystis genome database. The per gene was transcribed from 129 and 164 bp upstream of the translation initiation codon as 0.6-kb transcripts. The Per protein was detected a s a fusion protein with a molecular mass of 15 kDa by the epitope tag fusion method using a c-Myc epitope tag. Disruption of the per gene in wild-type cells shortened the period of the rhythms by 1 h, although it did not affect other properties of the rhythms, whereas its overexp ression extended the period by 3 h with a concomitant reduction in the amplitude of the rhythms, In various clock mutants examined, overexpr ession caused arrhythmicity. Thus, Per is likely to function as a modi fier of the circadian clock in Synechococcus.