Acclimation of chloroplast transglutaminase to high NaCl concentration in a polyamine-deficient variant strain of Dunaliella salina and in its wild type

The wild type (Wt) and the polyamine-deficient strain (PA(-)vs) of the halo tolerant Dunaliella salina were subjected to stress caused by 3.5 mol/L NaC l concentration. The chloroplasts were isolated and the molecular aspects o f their reaction to salt stress were studied together with their recovery r esponse to these hyper-saline conditions. In the Wt, the photosynthetic complexes were found to be severely affected by salt stress under light conditions. Transglutaminases, which are present in chloroplasts as two units of 25 and 50 kDa, were immunorecognized by an tibodies raised against rat prostatic gland transglutaminase. The amount, i n particular that of the 50 kDa unit, underwent an immediate change followi ng hyper-saline stress. These concentration changes were found to coincide with variations in enzymic activity, which is also affected by the presence or absence of light. The PA(-)vs has a concentration of proteins and chlorophylls which is much lower than that of the Wt. In addition, the PA(-)vs appeared to be more sev erely affected by both salt and subculture stresses. Its recovery time was also longer. Its TGase activity increased after salt stress and was always higher in the light than in the dark, except soon after subculture, showing an additive stress effect of salt and light. In the PA(-)vs acclimated to high salinity, or immediately after stress application, the chloroplast con tent of chlorophyll a and b was considerably enhanced, like the TGase activ ity (by two-fold or more), and these changes exhibited almost coincident be haviours. Some transglutaminase substrates (proteins of 68, 55, 29 and 27 kDa) were f ound to be similar to those present in higher plants (thylakoid photosynthe tic complexes and Rubisco). They were more markedly labelled by [1,4-C-14] polyamines when the transglutaminase assay was performed in the light than in the dark, and much more in algae already acclimated to hyper-saline cond itions than in those cultured in the optimal saline medium, or subjected to stress. The amount of 68 and 55 kDa polypeptides was particularly high in the 3.5 mol/L NaCl acclimated cells. The possible role of poly amine conjug ation in the assembly of chloroplast proteins in cells affected by salt str ess is discussed.