pheST operon dynamics and pheS gene duplication mediated genome stability and evolution in E. coli: evidence that fitC4 is an allele of pheS.

backgroundThis laboratory has been working on fit genes (factor involved in transcription) for many years. The fitA76 is temperature-sensitive (Ts) double mutant, carrying two lesions, pheS5 (pheS) and fit95 (pheT) with a combined defect at transcription. The fitB and fitC mutations were isolated as extragenic-suppressor(s) of fitA76. While fitA76 mapped at 38.7 min, fitC4 mapped at 39.01 min. The fitC4 and fitA76*, were two accompanied mutations found in temperature-insensitive derivative (JV4), isolated from fitA76 mutant. The fitC4 suppresses fitA76*(but not fitA76) and both are transcription defective. This work focuses on elucidating gene-identity of fitC4 locus, and its role in allele-specific genetic-suppression and genome-integrity.

methodsMutations were mobilized by P1-transductions. Marker-rescue analyses were done with Kohara-phages to narrow down the fitC4 location. Further mapping was done by complementation analyses employing various plasmids spanning that region. Complementation assays were quantitated by relative viability.

resultsAmong the Kohara phages/clones tested (321-330), the same clones 322&323 which earlier marker rescued fitA76/fit95 mutations, also marker rescued fitC4 mutation. But, fitC4 maps totally outside the overlapping region of two Kohara phages, tested positive. Complementation analyses by various plasmids spanning that region indicate that, fitC4 is same as pheS. Further, one of the clones bearing only pheT

conclusionThe fitC4 is same as pheS allele. Only pheS but not full pheST-operon is duplicated as fitC4. The pheS gene duplication could help in allele-specific genetic suppression and adaptation of its ability as transcription factor, thus contributing to genome-stability and genome-evolution.