Fig. 1. Growth, morphology and viability of the double rnhAB mutants.

A. A scheme of in vivo substrates of the two RNase H enzymes. The common substrate, framed in bright green, is the RNA-run with at least four contiguous rNs, which we call “R-tract”. HI and H1, HII and H2 refer to RNase H enzymes of prokaryotes and eukaryotes accordingly. B. Colony size on LB agar, 37°C, 24 hours. Strains: WT, AB1157; ΔrnhA, L-413; ΔrnhB, L-415; ΔrnhAB, L-416. C. Images of rnh and wild type strains stained with DAPI and observed by Hiraga's fluo-phase combined method. Cells were grown at 37°C in LB. The strains are like in “B”. D. Viability of the strains, determined as the ratio of the colony forming units (CFUs) to the microscopic counts in the same volume of the culture. Overnight cultures grown at 30°C were diluted and grown at the temperature (indicated by the first number) to OD 0.2-0.3 (about 2 hours), then cultures were serially diluted and plated on LB plates developed for 16 hours at the temperature indicated by the second number in pairs. Average viability (± SEM) of the eight WT measurements and six measurements for the rnhAB mutant cells is shown (the low titers of the two MG1655 ΔrnhAB cultures at 42°C were not used in the calculation). Strains: AB1157, L-416, MG1655, L-419. E. An enlarged image of the rnhAB mutant cells (processed as in panel C), to show nucleoids of both filamenting and normal-looking cells in some detail. F. Anaerobic growth inhibition of the rnhA and anaerobic lethality of rnhAB strains. Dilution-spotting of strains (like in “B”) was done in an anaerobic chamber on LB plates. Plates were incubated at room temperature in the chamber for 24 hours, then shifted to 28°C aerobic conditions for another 48 hours. G. The uvrA defect further reduces the colony size of the rnhAB double mutant. Strains: rnhAB, L-416; uvrA rnhA, L-414; uvrA rnhAB, L-417.