330 BACTERIOLOGICAL CHEMISTRY 



in a rhythmic order depending on their proportion in 

 the molecule. Other proteins, although built up of the 

 same amino acids, differ from silk fibroin and one another 

 in having different proportions of the amino -acids and, 

 therefore, different periodicity or internal structure. 



A certain amount of information is being obtained 

 as a result of studying the action of specific peptidases 

 which attack only peptide groups between particular 

 amino -acids. By this means it has been found possible 

 to determine whether or not certain pairs of amino-acids, 

 for instance, occur together in a given protein. Obviously, 

 considering the number of amino-acids available (about 

 twenty) and the number present in a protein, the number 

 of possible arrangements and, therefore, of possible 

 proteins is extremely large. It is to this great variety of 

 proteins that we owe many of the serological reactions 

 of bacteria, the reactions of antigens and antibodies 

 affording a means of detecting the subtle differences in 

 arrangement of the amino-acids which are at present 

 beyond the power of chemical methods. 



As was mentioned in Chapter V, the proteins of 

 bacteria and the yeasts are in the main very like those of 

 plants and animals, containing the same amino-acids in 

 much the same proportions, and falling into the globulin 

 or albumin groups, as these are determined by solubility 

 properties . 



An interesting recent development in connection with 

 the proteins of micro-organisms is the claim put forward 

 by Stanley that the virus of tobacco mosaic disease is a 

 crystalline protein. The protein can be isolated only from 

 diseased plants. Inoculation of healthy plants with as 

 little as 10~^^ g. of the crystals produces the disease and 

 gives rise to the production of large quantities of the 

 protein. It has a molecular weight, determined by 

 sedimentation in the ultracentrifuge and from its size 

 according to X-ray analysis, of about 17 million ; the 

 molecular weight of the normal proteins of the healthy 



