Small Cab-like proteins retard degradation of photosystem II-associated chlorophyll in Synechocystis sp. PCC 6803: Kinetic analysis of pigment labeling with ¹⁵N and ¹³C

Isotope (Na¹⁵NO₃, (¹⁵NH ₄)SO₄ or [¹³C]glucose) labeling was used to analyze chlorophyll synthesis and degradation rates in a set of Synechocystis mutants that lacked single or multiple small Cab-like proteins (SCPs), as well as photosystem I or II. When all five small Cab-like proteins were inactivated in the wild-type background, chlorophyll stability was not affected unless the scpABCDE^- strain was grown at a moderately high light intensity of 100-300 μmol photons m^-2 s^-1. However, the half-life time of chlorophyll was 5-fold shorter in the photosystem I-less/scpABCDE ^- strain than in the photosystem I-less strain even when grown at low light intensity (∼3 μmol photons m^-2 s^-1) (32 ± 5 and 161 ± 25 h, respectively). In other photosystem I-less mutants that lacked one to four of the scp genes the chlorophyll lifetime was in between these two values, with the chlorophyll lifetime generally decreasing with an increasing number of inactivated scps. In contrast, the chlorophyll biosynthesis rate was only marginally affected by inactivation of scps except when all five scp genes were deleted. Small Cab-like protein deficiency did not significantly affect photoinhibition or turnover of photosystem II-associated β-carotene. It is concluded that SCPs do not alter the stability of functional photosystem II complexes but retard the degradation of photosystem II-associated chlorophyll, consistent with the proposed involvement of SCPs in photosystem II re-assembly or/and repair processes by temporarily binding chlorophyll while photosystem II protein components are being replaced.