Nuclear Dependency of Chloroplast Proteins in Acetabularia

A membrane protein fraction, insoluble in water, has been prepared from Acetabularia chloroplasts. The greater part of these proteins could be dissolved in a phenol—formic acid—water mixture (2:1:1, v/v/v). The dissolved proteins were subjected to gel electrophoresis. The electrophoretic pattern was reproducible for one species. The protein patterns of A. calyculus and A. mediterranea were composed of 8 peaks common for both species and of at least 3 peaks which were species‐specific. By means of nuclear transplantation and implantation techniques it was shown that the species‐specific chloroplast‐proteins were nucleus‐dependent and therefore were coded by the nuclear DNA. Six weeks after the operation the patterns of the transplants and implants had changed into that of the nucleus donor species. It was concluded that the mRNA of the nucleus acceptor cell lost its stability under the influence of the implanted or transplanted nucleus. Both cycloheximide and chloramphenicol inhibited the incorporation of radioactive amino acids into the insoluble chloroplast‐protein fraction. The electrophoretic pattern of the labelled chloroplast‐proteins consisted of at least 3 peaks. Cycloheximide inhibited the incorporation into peak 3, whereas chloramphenicol affected the incorporation into the two other peaks. Isolated chloroplasts incorporated amino acids only into peaks 1 and 2. Peak 3 was absent in the pattern in vitro. The protein groups which were involved in the formation of chloroplasts were synthesized either by cytoplasmic 80 S or by 70 S chloroplast ribosomes. It was concluded that the autonomy of the chloroplasts was limited by the influence of the nucleus.