Effects of Δ<i>sqdB</i> and Δ<i>sqdX</i> in <i>Synechococcus</i> on cellular responses to S-starvation and on recovery of cell growth with re-supplementation of sulfate.

<p>Cell growth (a, OD₇₃₀ values), and the Chl (b) and phycobilisome (c, d) contents during S-starvation were investigated in the WT (open circles), and disruptants of Δ<i>sqdB</i> (open triangles) and Δ<i>sqdX</i> (open squares), respectively. The inset in (a) shows photographs of normally, weakly, and strongly bleached cultures of the WT, and Δ<i>sqdB</i> (Δ<i>B</i>) and Δ<i>sqdX</i> (Δ<i>X</i>) disruptants, respectively. (d) Cells were starved for S with illumination with light at a milder intensity (14 μmol photons m^-2s^-1) than usual [(c), 24 μmol photons m^-2s^-1]. The phycobilisome content was determined by spectroscopy (c; d, left) or SDS-PAGE of soluble fractions of mechanically disrupted cells (d, right). (e) Cells starved of S for 3 days were shifted to S-replete conditions for recovery from the retarded growth. Left, S-starved cells of the WT (white bars) and Δ<i>sqdB</i> disruptant (grey bars) were initially adjusted to 0.1 in OD₇₃₀ value with S-replete medium. Right, S-starved cells of the WT (white bars) and Δ<i>sqdX</i> disruptant (grey bars) were initially adjusted to an OD₇₃₀ value of 0.2 with S-replete medium. The two-fold higher initial value of OD₇₃₀ for the Δ<i>sqdX</i> disruptant relative to that for Δ<i>sqdB</i> one was to avoid possible photodamage in the Δ<i>sqdX</i> one, owing to its severe damages to cell growth and Chl accumulation (Fig 5A and 5B). Broken lines correspond to initial levels in the cultures. The inset in (e) shows photographs of 2-day cultures of the WT and Δ<i>sqdX</i> (Δ<i>X</i>). The values shown are the averages ± SD for three distinct groups of data. The significance of differences was evaluated by Student’s t test. *P < 0.05.