Molecular and Genetic Analysis of Two Closely Linked Genes That Encode, Respectively, a Protein Phosphatase 1/2A/2B Homolog and a Protein Kinase Homolog in the Cyanobacterium Anabaena sp. Strain PCC 7120

Reversible protein phosphorylation plays important roles in signal transduction. One gene, prpA, encoding a protein similar to eukaryotic types of phosphoprotein phosphatases PP1, PP2A, and PP2B, was cloned from the nitrogen-fixing cyanobacterium Anabaena sp. strain PCC 7120. Interestingly, a eukaryotic-type protein kinase gene, pknE, was found 301 bp downstream of prpA. This unusual genetic arrangement provides the opportunity for study about how the balance between protein phosphorylation and dephosphorylation can regulate cellular activities. Both proteins were overproduced in Escherichia coli and used to raise polyclonal antibodies. Immunodetection and RNA/DNA hybridization experiments suggest that these two genes are unlikely to be coexpressed, despite their close genetic linkage. PrpA is expressed constitutively under different nitrogen conditions, while PknE expression varies according to the nature of the nitrogen source. Inactivation analysis in vivo suggests that PrpA and PknE function to ensure a correct level of phosphorylation of the targets in order to regulate similar biological processes such as heterocyst structure formation and nitrogen fixation.