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The last time I wrote you was in the Fall of 1962, when I made some inquiry about a vacancy in your Department. Since then
I have spent a year in Germany and came back only recently to Canada. Since Jan. 1st, 1964 I am appointed to the Staff of
this Department. In the field of my recent interest: lysogeny in mycobacteria I encountered some most interesting problems,
which I should like to discuss with you.
I'd like to call the phenomenon which I am going to describe now: reciprocal conversion in a mycobacterial host-virus
system. As you may imagine, I have no evidence of the incorporation of a homologous segment of the host genome into the phage
genome, however, at present, this does not seem to be the most important question. Actually, from the two alternately possible
1. exchange of a chromosome segment between phage and host, and
2. inducing activity of the modified host genome, which in turn to conversion by the phage, "converts" the phage itself,
experimental evidence or at least
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my interpretation of experimental data are in support of the second mechanism. Preliminary results indicate, that profound
changes found in the genetic make-up of this phage are not of the known mutational nature; neither due to infection of a carried
prophage by a superinfecting phage, nor to recombination of carried and superinfecting phages.
Last year in Borstel[?], I isolated with Dr. Bonicke several mycobacteriaphages, which were initially polyvalent, lysed several
species of the genus mycobacterium. By deadaptation through serial passages in a single host organism, we obtained hereditary
stable, species-specific phages for (and within) the group of rapidly growing mycobacteria. With the exception of one phage,
which is not the subject of present study, none of the phages here have lost or changed their newly acquired specificity in
the course of 20 passages and are still strictly monovalent.
I started my work in Vancouver by attempting to lysogenize M. suceperatis[?], M. phlei, M. fortuitum etc. with our "virulent"*
phages. They proved unexpectedly good
* Virulence does not refer to the origin or past history of these phages (they might have been liberated prior to their isolating
from hypogenic mycobacteria into the soil, or even be produced in the course of enrichment of soil samples with mycobacteria);
it refers only to the fact, that they lyse all studied strains of a single species which are not lysogenic for them or related
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lysogenizing agents. Lysogenic organisms were picked from the residual growth in centers of massive lysis and were purified
by single colony transfers. The isolated phages*, on the other hand, were purified by simple plague transfers.
The lysogenic complexes thus established, showed with one exception, the characteristic picture known for other lysogenic
systems: i.e. production of phage and immunity to homologous phage. In each instance, conversion could also be observed in
as much as each lysogenic complex showed in addition to immunity against homologous wild type phage and "temperate"
derivatives thereof, a linked resistance to a related phage.
In the one exceptional complex, the isolated phage changed its host range very drastically in the course of its "reduction"
(?) (mutation?) to prophage. It became polyvalent, somewhat differently though from its Borstel ancestor.
In order to explain the changes one might have postulated a priori:
1. a host induced modification,
2. Host range mutation
3. Induction of prophage by superinfecting phage.
4. Recombination of prophage and superinfecting phage,
*I shall try to avoid as much as possible the use of the term temperate, since the converted phages possess not only an extended
host range, but similar lytic activity to that of their wild type ancestors.
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5. Recombination of phage and host genome,
6. Conversion" of phage due to previous bacterial conversion caused by it.
Ad 1. Most induced modification could be excluded easily and quickly:
a. In serial single plaque transfers on the original host never did the new phage lose its changed host range,
b. It was in no previous conflict with new host
Ad 2. If plates with serum efficiency on original and new host, if it is miaintained [sic] or purified on original host,
the relative E.O.P. being: 1!! This excludes mutation and favors an in toto conversion of phage population derived from this
Ad 3. If it were a case of prophage induction, the host, which carries the prophage would not be attacked by homologous phage
isolate (discounting the "[ . . .]" phenomenon). However, this is not the case. Attempts to isolate phages from
uninfected hosts did fail, so far.
Ad 4. Segregation into different plaque or host range types were
a) ever observed on the original host, however
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b. two plaque types with correlated host range differences were formed on new host. If the possibility of a second host range
mutation, selectively favored by the new host, can be excluded, this then would be a segregation due to recombination. Observations
show, that each type can give rise anew to the other one, too, to passage in the new host, but regains its uniformity by transfer
to the original host.
c. if the two plaque types on the new host were segregants due to recombination of superinfecting phages with prophage carried
by the new host, the parental segregant:
1. Would not lyse the new host, which then would carry this phage in the form of prophage, and conversely
2. It should be more active against sensitive strains of some species than the other plaque type phage. Nevertheless, both
types plate with similar efficiency on new host and relative E.O.P. of "segregants" on other strains of the species
is alike 1.
d. It is time, on the other hand, that one single transfer through the new host suffice to reduce dramatically the EO.P. of
converted phage for the original strain. (This may be reminiscence of the past history of this phage, since deadaptation
in Borstel followed exactly the same pattern). Thus there is a change in the relative E.O.P. as
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compared to previous 1) to[?] 10 [to the negative 4] with one plague type and 10 [to the negative 6] or less with the other.
Thus change is unidirectional, and original relative E.O.P. can be reinstituted[?] by propagation on the original host.
5. and 6. By exclusion of the better known genetic mechanisms, we are left with the probability of a phage-host genome interaction.
While there is no direct evidence for this, there are several lines of reasoning which might substantiate this claim:
1st: This complex, while reproducible, is nevertheless unique so far, since replacement of either host or phage results in
the formation of the conventional lysogenic complexes only with no concomitant changes in the host range.
2nd: No further mutations occur on the original propagating host, the phage establishes itself as a "stable phage strain"
for the host (with which it might or might have not exchanged a chromosomal segment). This is expressed in the uniform plaque
and host range type as opposed to that on propagation in the new host.
3rd: While the changes in these phages are
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reproducible in case lysogeny is established with same host, they differ strikingly from related complexes. Related complexes
show cross immunity and linked resistance to other phages. There is no immunity, however, against converted phages, even
such complexes are lysed -- and with same efficiency as their sensitive ancestors -- which carry the common phage ancestor
(the host being only a different strain of same species) The loss of homology, due to which no immunity can be conferred
by carried "identical" phage vapor related complexes, shows the extent of change.
The entire mechanism is both phage and host specific (refering [sic] to the formation of the complex), furthermore host dependant
(its stabilities being conserved only in the original host).
6. The frequency with which changes occur in the host range of converted phage is irreconcilable with mutation or recombination.
Each phage particle isolated from these lysogenic complexes has actually a changed host range, since E.O.P new host/original
host is I, when obtained from a simple plaque purified on the original host.
I am aware that these results are of preliminary nature. No further characterization of wild type phage and its derivates
can be done without investigating their serological properties and immunological relationships.
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Changes in the host organism, on the other hand, need also to[?] carefully be studied. Segregation into lysogenic and sensitive
organisms has been already obtained (I presume that the appearance of "spontaneous lysis" in lysogenic cultures. .
.to presence of sensitive cells in neighborhood of lysogenic organisms; frequency approx. 10 [to the negative six].
I don't know, if I am justified, however, I have high hopes, that apart from providing an appropriate tool for the analysis
of the genetic fine structure of mycobacteria as well as their viruses, this and similar other systems might prove appropriate
models to study in general the nature of virulence and temperateness and may eventually offer a new experimental approach
to the further elucidation of such as yet speculative questions as reciprocal genetic changes (involving or not exchange of
chromosomal segments) and their significance in the correlative evolution of bacteria and viruses. But this is enough from
philosophy and I may be wrong.
I hope I did not inconvenience you much with this long letter. And I rely very much on your help and criticism, for which
I say you thanks in anticipation
Very sincerely yours
Stephen E. Juhasz
P.S. Do you think, that due to the chromosomal location of prophage, cross lysogeny is of taxonomic value? I have some data
on cross lysogeny in species combined otherwise quite distinctly related (not more anyhow than Salmonella and Escherichia
or flagella and Escherichia). I wonder, what the value of it was!!