REGULATION OF THE SUBUNIT COMPOSITION OF PLASTIDIC GLUTAMINE-SYNTHETASE OF THE WILD-TYPE AND OF THE PHYTOCHROME-DEFICIENT AUREA MUTANT OF TOMATO BY BLUE UV-A-LIGHT OR BY UV-B-LIGHT/

The photomorphogenetic aurea mutant of tomato severely deficient in sp ectrophotometrically active phytochromes was used to study the light-r egulation of the single-copy nuclear gene encoding plastidic glutamine synthetase (GS-2; EC 6.1.3.2). The de-etiolation of dark-grown aurea mutant seedling cotyledons showed an obligatory dependency on blue lig ht. A limited red light-responsiveness of etiolated aurea cotyledons i s, however, retained as seen by the stimulation of both the GS-2 trans cript and protein level in the cotyledons of aurea seedlings during gr owth in red light. The subunits of the octameric GS-2 enzyme were repr esented by polypeptides with similar electrophoretic mobilities (polyp eptides a) in etiolated wild-type or aurea mutant cotyledons. GS-2 pro teins with similar apparent molecular masses were also seen in the cot yledons of red light-grown aurea mutant seedlings. In contrast, GS-2 p olypeptides with different apparent molecular masses (polypeptides a a nd b) were detected in the cotyledons of wild-type seedlings grown in red light. This difference indicates that the (post-translational) mod ification of tomato GS-2 subunit composition is mediated by the photor eceptor phytochrome. The illumination of etiolated wild-type or aurea cotyledons with UV-A- or UV-B-light light resulted in an increase in b oth the GS-2 transcript and protein level. Following illumination of e tiolated wild-type seedlings with UV-A-light, the relative proportion of the GS-2 polypeptides a and b was similar than upon irradiation wit h blue light but different than after exposure to UV-B-or red light, T his result suggests the involvement of a blue/ UV-A-light-specific pho toreceptor in the regulation of tomato GS-2 subunit composition.