BACTERIAL MUTANTS IN AMINO ACID TRANSPORT 611 
panied by any defect in the processes which utilize internally synthesized proline 
for protein biosynthesis. 
A systematic attempt has recently been made to find a difference in the amount 
of [14C}proline attached to the “cytoplasmic membrane” fraction of Tr*,,, and 
Tr-pro Strains, but the result has been negative. Although the clarification of the 
biochemical mechanism involved in transport is of greatest interest to us, some 
indirect and partial information may soon be obtained by the methods of microbial 
genetics and physiology. Among these, we look forward to: (1) chromosomal mapping 
of the transport genes; (2) the determination of the number of enzyme determining 
units (cistrons) of the chromosome involved in transport; and (3) measurement of 
the rate of phenotypic expression of the transport character in a zygote formed 
from a Tr~ recipient and Tr+ donor. 
As a by-product of the isolation of these mutants, control may be exerted over 
the internal level of the amino acid pool, which nearly equilibrates, at least for pro- 
line, with the concentration in the medium. At least one application is of current 
interest: the determination of the minimum concentration of internal amino acid 
needed for the net synthesis of RNA which occurs when bacterial cells are treated 
with chloramphenicol. . 
In summary, our studies on a bacterial mutant show that the same mutation 
which causes a defect in proline transport is responsible for loss of the capacity for 
rapid exchange of proline at zero degrees. The defect in the mutant strain is not 
located in any biochemical step involved in the biosynthesis of protein from internally 
synthesized amino acids. 
REFERENCES 
1M. Lupin, D. KEssEL, A. BUDREAU AND J. D. Gross, Biochim. Biophys. Acta, 42 (1960) 535. 
2 D. KESSEL AND M. LuBin, Biochim. Biophys. Acta, 51 (1962) 32. 
3 E.T. Botton, R. J. Britten, D. B. Cowie anp R. B. RosBerts, Carnegie Inst. Wash. Year- 
book, 57 (1957) 127. 
