We are investigating a series of temperature-sensitive mutants of Escherichia coli with mutations clustered in the aroE-strA region of the chromosome. These mutants stop growing at 44 C. and are killed by prolonged incubation at this temperature. Many show an altered sensitivity or resistance to one of four ribosome specific antibiotics. At least four have been shown to accumulate ribosomal subunit precursors at the elevated temperature. These mutants are temperature-sensitive in protein synthesis assays in vitro, and the temperature-sensitivity can be localized to a specific subunit in most cases. Altered ribosomal proteins have been detected in two-dimensional polyacrylamide gels of the subunit proteins for six mutants. We are currently mapping these mutants and are examining the relationship between the altered ribosomal protein structure and function in protein synthesis. We have several F' partial diploid strains which carry relatively short episomes containing the ribosomal protein genes clustered in the aroE-strA region. We are isolating the episomal DNA from these strains and are using it in a DNA-dependent protein synthesizing system. We have identified a number of the ribosomal proteins which have been formed in this system. We are presently examining the regulation of ribosomal protein gene expression by looking at the effects of ribosomal proteins, ribosomal RNA and various small molecules on both the relative transcriptional and translational levels in this coupled system.