Sexual agglutinin of mating-type minus gametes inChlamydomonas reinhardtii: II. Purification and characterization of minus agglutinin and comparison with plus agglutinin

The transcriptional program of synchronous gametogenesis in Chlamydomonas reinhardtii

Cells of Chlamydomonas reinhardtii undergo gametogenesis to produce sexually competent gametes under nitrogen-starved conditions. By using a synchronized system for gametogenesis of early G1 cells, several previously identified marker genes and 18 novel nitrogen-starved gametogenesis (NSG) genes isolated by macroarray analysis were placed into at least three temporal classes of expression. Early genes are induced transiently in the first 2 h after transfer to nitrogen-free medium. Middle genes are strongly induced between 3 h and 4 h after nitrogen removal, a time corresponding to the acquisition of mating competency, suggesting their involvement in the gamete program. Late genes are induced between 5 h and 8 h after nitrogen removal, a time after the completion of gametic differentiation, suggesting that they are not directly involved in the formation of sexually competent gametes. All of the 18 NSG genes examined are induced in both mating-type plus and minus gametes and about two-thirds of the genes are also expressed in the mitotic cell cycle, especially at S/M phases.

Isolation and characterization of Chlamydomonas temperature-sensitive mutants affecting gametic differentiation under nitrogen-starved conditions

Two conditional mutants of Chlamydomonas reinhardtii, dif-1 and dif-2, affecting gametic differentiation under conditions of nitrogen (N)-starvation, have been isolated. These mutant cell remain "vegetative" at the restrictive temperature (35 degrees C) in -N medium, as defined by assays of cell body-agglutinin and cell wall lytic enzyme activities in the soluble fractions of cell homogenates. Moreover, the mutants fail to form mating structures at the restrictive temperature, but do so at the permissive temperature (25 degrees C). Temperature-shift experiments show that mutant cells which have differentiated into gametes at 25 degrees C dedifferentiate into "vegetative" cells under N-starvation conditions after transfer to 35 degrees C, but differentiate again into gametes at 25 degrees C. Genetic analyses indicate that the dif-1 and dif-2 genes are recessive and unlinked to each other or to the mating-type locus; the dif-1 phenotype cosegregates with a conditional flagellaless phenotype expressed in both +N and -N medium at the restrictive temperature.