By Hans - editor Neurath

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Extra resources for The Proteins Composition, Structure, and Function. Volume 4

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Although the presence of A or B alone on the ribosomes may be due to incomplete synthesis of the multichain enzyme, additional studies are warranted before the significance of these observations can be critically evaluated. -galactosidase results do argue convincingly against the idea that this multichain enzyme is formed only subsequent to the release of the subunits from ribosomes. The data do not, however, rule out the possibility that the aggregation process consists of an interaction between unbound chains and ribosome-bound subunits resulting in the formation of the multichain enzyme on the ribosome.

If specific amino acid changes could be correlated with known nucleotide changes, and if several nucleotide changes could be induced in the same coding unit, it should be possible to assign certain nucleotides to definite relative posi› tions in some amino acid coding units. Two types of mutagenic agents have been employed in studies of this type: (a) base analogs that cause 18. GENETIC CONTROL OF PROTEIN STRUCTURE 27 nucleotide pairing errors during replication or are incorporated into DNA by pairing errors, or (b) agents that chemically alter nucleotides in intact DNA or RNA with the result that a different nucleotide is incorporated during replication.

The same is true for amino acid changes A » B -» C, in that the coding units for A and B, and for B and C must have two nucleotides in common, and those for A and C must have one or two nucleotides in common. Although it would appear that these restrictions might permit the assignment of specific relative nucleotide sequences for mutationally related coding units for different amino acids, the apparent degeneracy detected in in vitro coding studies is so exten› sive it prohibits any definite assignments.

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