~ in Department of Bacteriology. 2¢-January ,1993. | My dear Luca, I oa, : I thought you might like to know that 2 have obtained a spontaneously occurring Hfr strain. This appéared in 58-161/F+(Spicer) under the following peculiar circimstancés. *I have found that F+,but not F-,cultures maintained on inspissated=eggat 4° tend to die out slowly over a period of many months. Some weeks ago. I found that 58/Spicer,transduced to F+ and kept on egg as-usual,failed to yield growth when quite large sweeps of growth were transferred to broth, There was no evidence -of lysis or plaque formation. “I left the egg culture on the bench as a reminder to do something about it. About a week later .Ecnoticed. that-about 20 colonies had grown up on the area of confluent(but dead) previous growth. Several of these colonies were normally =F. . -Qne-gf -themjhowever, was Hfr. The recombination rate is about X 1000 normal; it does not transduce F+ to W677/F-(25 colonies .testedi&::compared with 25 colonies picked from a mixture ; with an Nfr culture of the same strain which gave 100% conversion under the same conditions); 25 Hfr X F- prototrophs appear to be F- (period of incubation not yet completed). In fact,it seems to behave exactly as your mustard-induced strain does. The interesting thing is that UV depresses the fertility of Hfr,a result which I must admit I anticipated. I have been thinking a lot about Hfr over the last few weeks and the more I think of itt the more it seems to me to fit in to the view that the F+ agent ts acts as agpnetic vertor. The theory now is as follows. F+ is a virus though not necessarily one which under any circumstances..is: lytic: like; a- phage, It is liberated by the -majority(or.all)of F+.cells during the early stages of the growth cycle,and. remains: adsorbed. to the surface’ of. the..cell3alternatdvely, it isc liberated: free but ds: very: rapidly: adsorbed: on to other: cells. .. When’ adsorbed by F--cells: it: is infectivec but. its stability: within,. -a'host cell depends. on a rather: delicate: physiological. balance so >. . that, in, the. case. of. certain F-: populations: only. a proportion’ of cells ~becomec infected, F+ being: discarded by,or dying: in,the remainders. : A- small . proportion: of; F+ agents from Nfr strains..become intimately: assoc- dated: with part, of: the. chromosome(or:.one. of: several. chromosomes) of the - host: cell, so that when: these: are- liberated the’ chromosome is:. carried - with, them,..c The effect, of. UV) ds. to. increase the propertdon. of; cells, in, ‘Which. the.Ps, agent: 1s assoelated: with; genetic.elements (since UV decreases - +. transduction at. the: same..time, as; it; inereases: recombination) .« In iHfti strains, there has, been; a; mutation of: Fe and. the..establishment, of a “new, relationship: between:idt: ands the: cellosp: that all. the. F+. agents. now: | become associated, with genetic elements; 1.e,: those F+;agents-which, in ONfr: ceils, become; ; tied: to. genettc; elements edther naturally er: as the result of the new equilibrium. induced. by. UV,can..be regarded. .as. phenocop- sfes of: the: genetic alteratdéon whiechiisvHfyr;.. The: Hfr virus ;however,is sénherently unstable ineF-:gells-so that {t;cannot. establish. itself toss: produce Hfr:prototrophg;: I have been moOst*intepested te hearn Jin: this “Goanéetion.that, in Salmonedja ‘transduction which ‘appears to: bé-definitely odue:to phage carriage. the recipient strain ‘ay remain phage “sensitive although genetically altered. It is also a fact that only a proportion ay eas wrth owe ope wy La ST Pea: ° te fe T ares L wo yet oan é Bo fede kh. & £ Ont of:ssensitive: cekis'expossd %o6°tenhperate’ phage establigh''a’ 1¥sopénie! °° relationship, withrit.: 1 feel: thatuF+ may: éxist-in’ various. %allel#e™ ise! forms and: that.ittes:stability within: F= ¢ellsi depends: onia verydéelieate rélationship between just-the ‘right Pecallele. and. the:right: phystological Stavecof;the-eelljc Hence:youn Fi ,strain whtch can he transduced.by’ 3< Waksman F+ bart not by iKl?2F+cand withsshbeh: 50%: prototrophsgre:F=: + l2and- my | 58/F+ strain. whichcis very spoorlystransdueed.to F+:asecoinpared:with W67?/Fe and uwithewhich;ontys30% pretetrophs ‘are: F+.< I-deintsknew what: you ‘think rofal] -thts.— these sane ‘deepowaters!..I have. probably cexpressed it: all iverycbadly. sIeforgot sto mention that my ‘theorycascto-:whyHfri, = reecoubination iis depressed:by -UV-is:that :sinee :all ithe: Pe uagents ‘are. already fassoelated :with sgeneti¢ elements the iproportion which do sos... .: cannot .be Increased : by :UV,;while -UVidepresses the number. of. vectors :which can -make qf fective ‘contact