Characterization of A Synechocystis Phytochrome

Abstract

Phytochromes are photoreceptors common to lower and higher plants. Plant phytochrome apoproteins autocatalytically attach phytochromobilin, a linear tetrapyrrole chromophore [1]. They regulate a wide variety of photomorphogenetic responses of plants to light [2]. The total sequence of the Synechocystis sp. PCC 6803 chromosome revealed the existence of a putative phytochrome gene (phy, ORF slr0473) [3]. The phy gene potentially encodes a protein (PhyS) that exhibits homology to phytochromes throughout its length, in particular in the N-terminal region including the chromophore binding site [4]. Moreover, the putative Synechocystis phytochrome shows distinct homology to sensory histidine kinases. Sensory histidine kinases and the so-called response regulators form the “two component systems” of signal transduction in bacteria [5]. Two component systems have been discovered more recently also in eukaryotes including components involved in the ethylene-induced signal transduction in Arabidopsis [6]. The Synechocystis chromosome encodes a number of putative response regulators and histidine kinases [3]. Several of these kinases contain stretches of amino acids with limited similarity to the N-terminal domain of phytochromes and to domains of the ethylene receptor protein of Arabidopsis (summarized in Table 1). We named these putative histidine kinases “phytochrome like proteins” (abbreviated: plp) [7]. We disrupted the gene of one of these plp’s (plpA, encoded by ORF sll1124 [3]) by directed insertional mutagenesis. The obtained mutant was found to be unable to grow under blue light [7].