The program of protein synthesis during heterocyst differentiation in nitrogen-fixing blue-green algae

Abstract

The program of protein synthesis that accompanies cellular differentiation following transfer of the blue-green alga Nostoc muscorum from nitrogen-containing to nitrogen-free medium has been determined by polyacrylamide gel electrophoresis of whole cell proteins labeled with 35SO 4 = during successive intervals of the differentiation. Differentiating cells (proheterocysts, which become heterocysts) are distinguished from vegetative cells on the basis of the latter's susceptibility to lysis with lysozyme. At least ten sets of proteins can be distinguished on the basis of the time at which they are synthesized or the type of cell in which they are located. Regulation of most of these sets can be accounted for by classical induction or repression involving NH 4 + or a simple derivative of NH 4 +. An additional mechanism is required to explain how the synthesis of several sets of proteins is initiated in all cells following transfer to nitrogen-free medium, but is permitted to continue only in developing proheterocysts. The structural polypeptides of the nitrogenase enzyme complex are members of the latter set. In differentiated filaments, very few proteins are synthesized in both vegetative cells and heterocysts. The qualitatively different pattern of protein synthesis is established very early, within the first 9 hr after transfer. Moreover, the proteins present in proheterocysts at that time are already qualitatively different from those of vegetative cells. Rapid turnover of vegetative cell proteins appears to be a characteristic of the early development of proheterocysts.