Sexual agglutination in Chlamydomonas eugametos before and after cell fusion.

A new study of sexual agglutination between Chlamydomonas eugametos gametes and between vis-à-vis pairs has been made using techniques that allow one to distinguish between the flagella or cell bodies of individual mating types (mt(+) or mt(-)). It is shown that before mt(+) and mt(-) gametes fuse in pairs, their flagella, which adhere over their whole length, are maintained in a particular conformation around the mt(-) cell body. In clumps of agglutinating gametes the cells are asymmetrically distributed with the mt(+) gametes constituting the outer surface of the clumps with the mt(-) gametes on the inside. The flagella are then all directed towards the middle of the clump. This orientation of the flagella is maintained for approx. 8 min after cell fusion before the vis-à-vis pair becomes motile. At this stage, all the flagellar tips are activated. The original mt(+) flagellar tips then deactivate and swimming is resumed. The original mt(-) flagella remain immotile and activated after cell fusion and eventually shorten by a third, but only 30 min or more after fusion. Motile vis-à-vis pairs eventually settle to the substrate when the gamete bodies fuse completely to form a zygote. Settling vis-à-vis pairs are attracted to those that have already settled, to glutaraldehyde-fixed pairs and to flagella isolated from mt(-) gametes. They are not chemotactically attracted, rather they are weakly agglutinated. Living vis-à-vis pairs can be shown to aggregate in rows with the cell bodies lying side by side. It is argued that the flagellar agglutination sites involved in gamete recognition are also involved in vis-à-vis pair aggregation.

Dual pathways of glycerol assimilation in Klebsiella aerogenes NCIB418: their regulation and possible functional significance.

Klebsiella aerogenes NCIB 418 assimilates glycerol via alternative pathways: one involves a glycerol kinase with a high affinity for glycerol (apparent Km = 1-2x10(-6)M), and the second a glycerol dehydrogenase with a much lower affinity for its substrate (apparent Km=2-4x10(-2)M). In variously-limited chemostat cultures, one or the other pathway predominated. Thus, aerobic carbon-limited organisms contained only the glycerol kinase pathway whereas aerobic sulphate-limited or ammonia-limited organisms (grown on glycerol) used only the glycerol dehydrogenase pathway. Anaerobic cultures invariably contained glycerol dehydrogenase, and glycerol kinase was absent. Washed suspensions of aerobically-grown organisms oxidized glycerol with kinetics similar to that of the particular enzyme (the primary enzyme of the assimilatory pathway) which they possessed, thus indicating a close association between these two enzymes and the uptake process. But a supply of exogenous glycerol was not a prerequisite for the synthesis of either glycerol kinase or glycerol dehydrogenase, and nor was molecular oxygen the key factor in effecting modulation between the alternative pathways of glycerol metabolism, as had been previously suggested. The physiological significance of dual pathways of glycerol assimilation is discussed.