IMPORT INHIBITION OF POLY(HIS) CONTAINING CHLOROPLAST PRECURSOR PROTEINS BY NI2+ IONS

The precursor of the small submit of ribulose-1,5-bisphosphate carboxy lase (pSS) and a modified pSS containing a C-terminal hexahistidyl tai l (pSS(His)(6)) were imported into isolated Chlamydomonas chloroplasts with comparable efficiency. In the presence of Ni2+ ions the import o f pSS(His)(6) was inhibited and the precursor bound to the envelope re mained protease sensitive, while import of pSS was not affected. Addit ion of an excess of L-histidine suppressed the inhibition demonstratin g that the hexahistidyl-Ni2+ complex was responsible for import inhibi tion. Inhibition could be observed between about 0.5 and 10 mM Ni2+, d epending on the total protein content in the assay. Import incompetent Ni2+-precursor complexes can be used to study early events in chlorop last protein import.