The production of some nonproteinous, and lack of production of other proteinous, amino acids in model prebiotic synthesis, along with the instability of glutamine and asparagine, suggest that not all of the 20 present day proteinous amino acids gained entry into proteins directly from the primordial soup. Instead, a process of active co-evolution of the genetic code and its constituent amino acids would have to precede the final selection of these proteinous amono acids.

The tryptophan-auxotrophic Bacillus subtilis LC33 mutant strain utilizes either tryptophan or 4-fluorotryptophan for growth. Proteins therefore could be isolated from these cells in either tryptophan-containing or 4-fluorotryptophan-containing forms. Since 4-fluorotryptophan is non-fluorescent, tryptophan fluorescence would be suppressed in the 4-fluorotryptophan-containing proteins, facilitating the investigation of other chromophores either on the proteins or interacting with the proteins. This approach, potentially applicable to any protein endogenous to or clonable into B. subtilis, was illustrated by an examination of the fluorescence of B. subtilis ribosomal proteins.

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Properties of the ribosome-bound ,B-galactosidase were examined in Escherichia coli cells after prolonged induction. This fraction of enzyme was not chased from ribosomes by removal of inducer, or by treatments with hydroxylamine, puromycin, chloramphenicol, and azide. However, the metabolic turnover of this fraction could be demonstrated by means of a pulsed exposure to the phenylalanine analogue d-2thienylalanine, and this fraction was enriched in heavy forms relative to the soluble enzyme. These observations indicated a tight coupling of the release of ribosomebound enzyme to nascent enzyme synthesis, and it is suggested that the ribosomebound enzyme is related to an intermediate stage in the assembly of quarternary enzyme structures.The existence of ribosome-bound fl-galactosidase in Escherichia coli is well established (2,5,14). Moreover, on the basis of their biosynthetic behavior, Zipser has distinguished between two separate fractions of the ribosome-bound enzyme. A chaseable fraction appeared rapidly upon induction, and disappeared after the removal of inducer. A nonchaseable fraction accu-