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Metabolism of tRNA in near-ultraviolet-illuminated Escherichia coli. The tRNA repair hypothesis.

The relA+-dependent stringent response is an important component of the mechanism of the near-ultraviolet-induced growth delay. However, the behaviour of the intracellular level of ppGpp is unexpected [Thomas et al. (1981) Eur. J. Biochem. 118, 381-387] and this led us to examine the metabolism of tRNAs during the illumination period and the growth lag that follows. Analysis of the gel electrophoresis migration profiles of tRNA molecules, synthesized prior to the illumination period, provides no evidence for tRNA degradation. Rather, it is suggestive of the rearrangement of some cross-linked tRNA species during the growth lag. By the same technique the neosynthesis of one or several tRNA species escaping the stringent response could be ruled out at the beginning of the growth lag. The behaviour of the cross-linked tRNAs was followed by a chromatographic procedure allowing the quantitative evaluation of the 8-13 link present in vivo. Upon illumination of growing cells, one observes an initial linear increase of the 8-13 link content. Unexpectedly this is followed during the illumination period by an abrupt decrease. The 8-13 link content then remains stable. The data above suggest that part of the 8-13 link (25-40%) is eliminated from tRNA without degradation of the molecules involved. A tRNA repair hypothesis is proposed: elimination of the 8-13 link would occur by scission of the N1-C1' glycosidic bonds at positions 8 and 13 of tRNA. It would be followed by reinsertion of uracil and cytosine in their respective positions.