Analysis of the Escherichia coli nusA10(Cs) allele: relating nucleotide changes to phenotypes.

The Escherichia coli nusA gene product, known to influence transcription elongation, is essential for both bacterial viability and growth of lambdoid phages. We report the cloning and sequencing of the conditionally lethal nusA10(Cs) allele. Changes from nusA+ were observed at nucleotides 311 and 634. Functional studies showed that both nucleotide changes are necessary for the cold-sensitive phenotype, although bacteria with the change at 634 grew more slowly at 30 degrees C than the nusA+ controls. The mutant nusA10(Cs) gene product is not as active as nusA+ in supporting transcription antitermination mediated by the N regulatory protein of bacteriophage lambda. The change at nucleotide 634 is shown to be solely responsible for this phenotype. Four differences were found between the nusA+ gene that we sequenced and the published nusA sequence. These changes alter the reading frame of nusA in a functionally important domain [as identified by the nusA1 and nusA11(Ts) mutations], resulting in an arginine-rich region that may be involved with RNA binding.