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Journal of Bacteriology, June 2002, p. 3176-3185, Vol. 184, No. 12
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.12.3176-3185.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Atypical Processing in Domain III of 23S rRNA of Rhizobium leguminosarum ATCC 10004^T at a Position Homologous to an rRNA Fragmentation Site in Protozoa

Institut für Mikrobiologie und Molekularbiologie der Justus-Liebig-Universität Giessen, 35392 Giessen, Germany

Received 27 August 2001/ Accepted 8 March 2002

For still unknown reasons, the 23S rRNA of many -Proteobacteria shows a unique fragmentation pattern compared to other bacteria. The 23S rRNA processing involves RNase III and additional, yet unidentified enzymes. The -proteobacterium Rhizobium leguminosarum ATCC 10004^T possesses two fragmentation sites in its 23S rRNA. The first one harbors an intervening sequence in helix 9 which is cleaved by RNase III. We demonstrate that the mature 5' end of the resulting 2.6-kb rRNA fragment is generated by additional removal of helix 10. A fraction of the 2.6-kb rRNA is further processed in domain III, giving rise to two 1.3-kb rRNA fragments. We mapped the domain III fragmentation site and found it to be at a position which has only been reported for trypanosomatid protozoa. This fragmentation site is also unique in that it lacks an intervening sequence. We found that the simultaneous occurrence of 2.6-kb and 1.3-kb rRNA fragments is not due to interoperonal sequence differences but rather reflects slow processing. The different characteristics of the two fragmentation sites in the 23S rRNA suggest that they are processed by different mechanisms. Interestingly, the amount of 2.6-kb rRNA varies during culture growth. We observed a transient increase in the relative amount of 2.6-kb rRNA fragments during the first hours after inoculation, which points to changes in the ratio of rRNA synthesis rate to domain III processing rate during the growth of a culture.

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