section through a spore. The micrograph 

 clearly shows the large basophilic cap with 

 its finely granular contents; note also that 

 there is no material of a similar nature outside 

 the cap. It also shows the nucleus, the nucleolus, 

 and, although it looks like two, the single large 

 mitochondrion with the flagellum coming out 

 through the bottom. 



Figure 3 is a tangential section through the 

 nucleus showing partial sections of the nucleus 

 and the cap with the small osmiophilic particles 

 in it. I'm sure you've guessed that these are 

 ribosomes. That is what we think they are. The 

 pores in the nuclear membrane are particularly 

 obvious. K you look carefully at the membrane 

 of the cap itself, on the other hand, it is seen to 

 be continuous. There is a single large mito- 

 chondrion per spore; it is always acentrically 

 located and specifically associated with lipid 

 granules and what we suspect may turn out to be 

 polysaccharide granules. Figure 4 is a section 

 through the lower end of the spore showing the 

 centriole, or basal body, at the base of the 

 flagellum, going through the mitochondrion in 

 a small channel. Figure 5 is a section taken at 

 right angles to the last and shows quite clearly 

 that the flagellum doesn't just pass by the mito- 

 chondrion; it goes right through a channel in the 

 middle. Note the direction of the flagellar shaft 

 with respect to the mitochondrion and the 

 "rootlets" that extend down through other chan- 

 nels into the body of the mitochondrion itself. 

 We don't have much of an idea concerning the 

 function of these at present. The purpose of this 

 hasty survey of zoospore morphology has been 

 to emphasize the highly organized state of the 

 spores; the nearest approximation I can think 

 of would be a protozoan or perhaps a sperm 

 cell. It is obvious that a considerable intra- 

 cellular transformation must occur in the proc- 

 ess of forming these spores at the end of the 

 life cycle. 



Several years ago, Dr. Gilbert Turian, in 

 Switzerland, demonstrated the presence of these 

 small particles in electron micrographs of 

 Allomyces nuclear caps (2), but Allomyces is a 

 filamentous organism and is, therefore, difficult 

 to grow in synchronized cultures. We have been 

 interested in looking at the caps in B. emersonii. 

 partly because of familiarity with the orga- 

 nism, but partly because, in terms of differentia- 

 tion, we can get much better synchrony for our 

 work. 



Figure 6 shows that one can readily isolate 

 the nuclear caps from zoospores. Because the 

 caps were central to our interests, we wanted 



to be sure we could get them out, characterize 

 the contents, and be certain they were what we 

 thought they were, that is to say, ribosomes. By 

 proper procedures, one can gently rupture the 

 spores, separate the caps, and purify them by 

 differential centrifugation (3). When we did this, 

 we found that they were composed of about 40% 

 RNA, 60% protein, and made up about 70% of 

 the total RNA of the cell. They also contributed 

 something like 18% of the total dry weight of 

 the spore. This was all consistent with a ribo- 

 somal composition. 



Analysis of the purified caps demonstrated 

 that they matched ribosomes in their chemical 

 composition. When we purified the caps and then 

 isolated their particulate contents, we could also 

 show that they were ribosome-like by examining 

 them in the analytical ultracentrifuge to obtain 

 their sedimentation coefficient. They sediment 

 at about SOS, a value similar to that found for 

 other fungi. The particles also dissociate in low 

 magnesium ion concentration and contain a latent 

 ribonuclease. Thus, they appear to have the 

 characteristics of ribosomes as determined by 

 a variety of procedures. They also seem to be 

 very pure. When the particles are isolated from 

 caps, one obtains a glassy pellet so transparent 



Fig. 3. 



Electron micrograph of a section through a zoospore 

 tangential to the nucleus. Ig, Upid granule; pg, poly- 

 saccharide; g, granule. 



166 



