SpdA, a putative phosphodiesterase at the cyaD1 locus. (A) Genetic map of the cyaD1 locus on the S. meliloti chromosome. Arrows indicate the direction of transcription. (B) SpdA has the five conserved subdomains (boxed) of class III phosphodiesterases. Sequence alignment of SpdA with cyclic adenosine monophosphate phosphodiesterases from Escherichia coli (CpdA), Mycobacterium tuberculosis (Rv0805) and Haemophilus influenzae (Icc) and S. meliloti. The invariant amino acids forming the metal ion binding sites of class III PDEs are marked with (#). Alignment was made using ClustalW algorithm .