Fig. 7. The effect of GbpD overexpression on the adhesion strength of mutants with undetectable cyclic nucleotide levels. The adhesion strength of several mutants was studied by measuring the percentage of cells that became detached from the substrate after shaking for 60 minutes at 150 rpm. The requirement of cGMP for GbpD function was studied by overexpressing GbpD in a guanylyl cyclase double knockout, sgc^–/gca^–; cGMP levels of this cell line were below the detection limit (0.019 pmol/10⁷ cells or 3 nM). To study the requirement of cAMP for GbpD function, the phosphodiesterase GbpB was overexpressed. This led to cAMP levels that were below the detection limit of the assay (0.024 pmol/10⁷ cells or 4 nM). Finally, GbpD was overexpressed in a cell line that was devoid of cAMP and cGMP by overexpressing GbpB protein in the sgc^–/gca^– cell line (both cGMP and cAMP levels were below the detection limit). Error bars represent the s.d. of at least two independent experiments.