Section 3 — Molecular and Microbial Genetics 



that all information necessary for the secondary 

 and tertiary structure of this enzyme is contained 

 within the primary amino acid sequence. More 

 recently, similar results have been obtained 

 with other proteins, indicating that this conclu- 

 sion may be generally applicable. Because of 

 the relatively long time required for the refolding 

 process in vitro, in contrast to the rapid synthesis 

 of native proteins in vivo, a catalytic system was 

 sought and found in rat liver extracts. This 

 system, consisting of soluble microsomal and 

 supernatant factors, greatly accelerates the reac- 

 tivation of reduced ribonuclease. 



In order to study the effects of amino acid 

 alterations on the configuration, as opposed to 

 the "active center", of proteins, chemically 

 modified, but still active, enzymes have been 

 prepared. The effects of the modifications on 

 folding of the extended molecules (following 

 reduction) are now being studied. 



Much of this work was done in conjunction 

 with Dr. Christian B. Anfinsen. 



3.8. The Evolution of the Fibrinogen Molecule' 



Russell F. Doolittle and Birger Blomback 

 (Stockholm, Sweden). 



Fibrinogen is a protein molecule which is 

 apparently found in the blood plasmas of all 

 vertebrate species. During its conversion to 

 fibrin by thrombin, certain segments of the 

 parent molecule are proteolytically released. 

 These peptides, usually termed the fibrinopep- 

 tides A and B, range from 13 to 21 amino acids 

 in length, depending on the species from which 

 they are obtained. Comparisons of the amino 

 acid sequences of fibrinopeptides from a variety 

 of vertebrates has permitted us to follow a 

 flow of amino acid substitutions through these 

 regions of the fibrinogen molecule. Complete 

 sequences are now available for the A and B 

 fibrinopeptides from ox, human, sheep, and pig. 

 In addition, partial amino acid sequences of 

 the peptides from several other mammals and 

 the lamprey eel (Petromyzon marinus) are re- 

 ported. The differences in sequence are greater 

 than expected. Thus, the B fibrinopeptides of 

 sheep and ox differ in ten of twenty residues. 

 On the other hand, certain positions and seg- 

 ments of the peptides appear to have been 

 maintained throughout all the species examined. 

 Only one fibrinopeptide has been isolated from 

 lamprey eel fibrinogen so far. Its partial amino 

 acid sequence suggests that it has characteristics 

 of both the A and B mammalian fibrinopeptides. 



For example, it contains tyrosine-O-sulfate, an 

 unusual amino acid found in the B fibrinopep- 

 tides of several mammals, but its C terminal 

 portion corresponds to the C terminal sequence 

 of the mammalian A fibrinopeptides. Although 

 our observations are restricted to those amino 

 acid substitutions which have survived, as op- 

 posed to all those that may have occurred, it 

 is possible to draw some conclusions about the 

 plausibility of suggested amino acid codes. 



3.9. Control of DNA Replication in an Organism 

 with Synchronous Mitosis. E. Guttes and 

 S. Guttes (Providence, U.S.A.). 



The nuclei of the plasmodial slime mold, 

 Physarum polycephalum, divide in synchrony 

 every 12-14 hr. The replication of DNA occurs 

 during a period of 3-4 hr immediately following 

 mitosis. The role of the nuclei in the initiation 

 of this process was investigated by examining 

 their competence for DNA replication at various 

 stages of the replication cycle. For this purpose, 

 the normal correlation between the nuclear and 

 the cytoplasmic replication cycle was disrupted 

 by implanting nuclei representing one stage of the 

 cellular cycle into Plasmodia representing an- 

 other. We found that hostplasmodia at a stage 

 of the cell cycle at which they are able to sus- 

 tain DNA biosynthesis and replication, cannot 

 initiate DNA replication in implanted nuclei, 

 unless these have undergone mitosis just prior 

 to the implantation. This failure is not due to a 

 lack of functional integration between the host 

 Plasmodium and the implanted nuclei, nor to 

 unspecific damage to the latter as a result of the 

 implantation procedure. DNA feeding with 

 subsequent exposure to tritiated thymidine re- 

 sulted in strong DNAse labile cytoplasmic la- 

 belling at all times of the intermitotic period. It is 

 concluded that only at mitosis the nuclei acquire 

 competence for DNA replication and that this 

 event is a critically controlling factor in the 

 timing of DNA replication. The structural 

 alteration of the nuclei in acquiring of compe- 

 tence is not reflected in their in vitro behaviour. 

 In an experiment involving exposure of fixed 

 nuclei to a suitable DNA-polymerase prepara- 

 tion* 1 ) we found that nuclei in ethanol-fixed 

 smear preparations were at no time of the repli- 

 cation cycle able to serve as primers, whereas 

 nuclei which were previously fixed with denatur- 

 ing ethanol-acetic acid (2 > were able to serve as 

 primers. An investigation with the electron 

 microscope^ 3 ) has shown that in vivo the onset of 

 DNA replication coincides precisely with the 

 time at which prenucleolar material is beginning 



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