A SIGMA-FACTOR THAT MODIFIES THE CIRCADIAN EXPRESSION OF A SUBSET OF GENES IN CYANOBACTERIA

We isolated mutants affected in the circadian expression of the psbAI gene in Synechococcus sp. strain PCC 7942 using a strategy that tags t he genomic locus responsible for the mutant phenotype, The search iden tified one short period (22 h) mutant (M2) and two low amplitude mutan ts, one of which showed apparent arhythmia (M11) and one that was stil l clearly rhythmic (M16). We characterized the disrupted locus of the low amplitude but still rhythmic mutant (M16) as the rpoD2 gene, a mem ber of a gene family that encodes sigma(70)-like transcription factors in Synechococcus, We also inactivated rpoD2 in a number of reporter s trains and showed that the circadian expression of some genes is not m odified by the loss of this sigma factor, Therefore, we conclude that rpoD2 is a component of an output pathway of the biological clock that affects the circadian expression of a subset of genes in Synechococcu s, This work demonstrates a direct link between a transcription factor and the manifestation of circadian gene expression.