A single base mutation was constructed at position 1538 ofEscherichia coli 16S rRNA, changing a cytidine to a uridine. This position is in the Shine-Dalgarno region, thought to be involved in base-pairing to mRNA during initiation ofprotein synthesis. The mutation was constructed by using a synthetic oligodeoxynucleotide that differs in sequence by one base from the wild-type sequence of 16S rRNA. This oligonucleotide was used as a primer on single-stranded DNA of phage M13, into which was cloned a specific region of DNA encoding 16S rRNA. The mutation is lethal when expressed from the normal promoters of rRNA operons, PI and P2, in a high-copy-number plasmid. Expression can be repressed by a temperature-sensitive repressor, c1857, in combination with the bacteriophage XPL promoter. Induction of transcription by temperature shift yields mutant 16S rRNA that is processed and assembled into functional ribosomal subunits. The presence of mutant ribosomes retards cell growth and dramatically alters incorporation of [35S]methionine into a large proportion of the cellular proteins. The change in level of synthesis of individual proteins correlates with the change in base-pairing between mutant rRNA and the Shine-Dalgarno region of the mRNA.

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Serum Na + and osmolality increased significantly within 12-24 hours after transfer of Fundulus heteroclitus to seawater (SW) but returned to freshwater (FW) levels after 4 days in SW. Na+K'-adenosine triphosphatase (ATPase) remained low during the first 2 days in SW but increased to a level characteristic of fully acclimated fish within 4 days. Serum cortisol did not display a marked elevation during the rise in Na+K+-ATPase. Thus, the pattern of SW-acclimation in E: heteroclitus is similar to that of the eel (Anguilla rostrata), but acclimation is accomplished somewhat more rapidly in l?heteroclitus and the prolonged period of elevated serum cortisol observed in the eel does not occur.A distinct peak in cortisol was observed during acclimation to FW when gill Na+K+-ATPase and serum osmolality were declining. This indicates that elevated serum cortisol does not always lead to an increase in Na"K+-ATPase activity.