Gametic differentiation of the unicellular green alga Chiamydomonas reinhardtii proceeds in two steps controlled by the extrinsic signals nitrogen deficiency and light. Nitrogen deprivation induces the differentiation of vegetative cells to sexually immature pregametes. A light signal is required to convert the pregametes to gametes. Both signals are also required for the maintenance of mating competence. Two converging signal transduction chains are proposed to control gamete formation. For the differentiation of pregametes to gametes, a fluence ratedependent reaction, requiring continuous irradiation, is suggested by photobiological experiments.By gametic differentiation, Chlamydomonas reinhardtii vegetative cells are transformed into mating-competent cells. This differentiation has been shown to be associated with changes in the cell's biochemistry and subcellular morphology. The most prominent changes at the molecular level have been demonstrated for the ribosomes. Cytoplasmic and chloroplast ribosomes decrease in number (7,8,13) and new ribosomes appear that exhibit decreased translational accuracy (2), altered susceptibility to antibiotics (9), and ribosomal proteins with different electrophoretic mobilities (9). Morphologically, the acquisition of two new organelles, a mating structure and a special type of Golgi-derived vesicle are most prominent (3, 7). These and other changes, e.g. the appearance of sexual agglutinins (1,11,19) and the synthesis of gametic autolysin (12,16) suggest that gametic differentiation is based on changes in the pattern of gene expression. Characteristic differences in the rates of synthesis of individual proteins (15)

Transcriptionally active chromosomes (TACs) were isolated from mature chloroplasts of barley, from proplastids enriched in basal segments of barley primary foliage leaves, and from ribosome-deficient plastids of heat-bleached barley leaves. Immunological analysis with a specific antibody raised against the plastid rpoA gene product revealed that chloroplasts contain an immunoreactive protein of 38 kDa in the TAC fraction which appears to be identical to the alpha-subunit contained in the soluble RNA polymerase (sRNAP) fraction of the same chloroplasts. However, only traces of immunoreactive protein were detected in a TAC preparation derived from "proplastids". A positive correlation could be demonstrated between transcriptional activity and the amount of immunoreactive 38-kDa protein by analyzing different TAC fractions eluting at different times during gel filtration of a standard TAC preparation as well as in TAC preparations obtained under various detergent conditions.