454 



( ii \i> i i r 35 



corporated into the progeny of </>R17.'" an 

 RNA phage. 



Genetic DNA rran8criptio P Genetic RNA 



The mate-killer (ma) particle in Parame- 

 cium, like the similar bacterial endosymbi- 

 otes lambda and kappa (see p. 374), de- 

 pends upon the micronuclear genes of its 

 host for its maintenance. In this case, two 

 unlinked dominant genes, M, and Af.. are 

 involved, either one independently capable 

 of supporting growth and replication of the 

 mu particles. Sensitive, non-mate-killer in- 

 dividuals containing either A-/,, M-, or both 

 do not spontaneously generate mu particles, 

 so these genes do not form mu particles di- 

 rectly. If after conjugation of a mate-killer, 

 its M dominant genes are replaced and the 

 resulting exconjugant is m, m, /??•_. ra L ., the 

 mu particles (which are visible and contain 

 DNA) and the mate-killer phenotype are 

 lost some 8 to 1 8 fissions later. This de- 

 layed loss of mu particles is abrupt, since 

 a cell has either a large number of particles 

 or none. Consequently, it is suggested that 

 the M genes control mu particle existence 

 by their gene products, called metagons. 



Numerous tests of this hypothesis yield 

 confirmative results. It has also been found 

 in Paramecium that: 



1 . A single metagon is sufficient to sup- 

 port numerous mu particles 



2. Metagons rarely, if ever, replicate 



3. Metagons can be transferred via a cy- 

 toplasmic bridge from one member of 

 a conjugating pair to the other 



4. In the absence of an M gene, the met- 

 agons are diluted in successive fissions 



5. Normally, about 1,000 metagons are 

 present in each mate-killer individual. 



Moreover, since ribonuclease destroys them, 

 RNA is an essential constituent of metagons. 

 Mu particles are destroyed exactly one fis- 



10 See D. B. Ellis and W. Paranchych (1963). 



sion after the metagons are eliminated from 

 a cell by ribonuclease. Metagons can be 

 synthesized two fissions after ribonuclease 

 treatment, provided an M gene is present." 

 Subsequent evidence shows that metagons 

 are messenger RNA with a high proportion 

 of G -f- C. Accordingly, the existence of 

 an endosymbiont is regulated by its host's 

 messenger RNA. Not only can other para- 

 mecia be infected with metagons, but the 

 very different protozoan Didinium can ac- 

 quire both metagons and mu by eating par- 

 amecia which contain them. Metagonic 

 RNA recovered from Didinium or parame- 

 cia can hybridize with DNA from M-con- 

 taining paramccia and, to a lesser extent, 

 m-containing paramecia but not with DNA 

 from Didinium. Therefore, we conclude 

 that Didinium contains no M genes. Never- 

 theless, the metagons not only persist but 

 multiply in Didinium. 1 - These results sug- 

 gest that in Paramecium, metagonic RNA 

 is somehow inhibited from replicating al- 

 though it persists as a messenger for a rather 

 long time. In this respect the RNA meta- 

 gon resembles the DNA in an abortive 

 transduction. In Paramecium, the RNA 

 metagon is behaving like an RNA virus 

 generated but incapable of replication; in 

 Didinium, like one incapable of being gen- 

 erated but capable of being replicated. It 

 is important for us to learn as much as pos- 

 sible about the nature and origin of the met- 

 agon-replicating enzyme and the mechanism 

 that inhibits or prevents metagon replica- 

 tion under particular circumstances. A 

 host's genetic control of RNA replication 

 may sometimes involve the action of inter- 

 ferons — proteins (presumably synthesized 

 through the intermediary functioning of mes- 

 senger RNA) which prevent replication of 

 certain viruses. 1 ' 5 



11 The preceding discussion is based upon work of 

 I. Gibson and G. H. Beale ( 1963). 



12 See I. Gibson and T. M. Sonneborn (1964). 

 1! See R. Z. Lockhart, Jr. (1964). 



