Senescence associated with the over-replication of a mitochondrial retroplasmid in Neurospora crassa

Mitochondrial DNA rearrangements and deletions are a prevailing feature of filamentous fungal cultures that undergo senescence. In Neurospora spp., strains containing the Mauriceville and Varkud mitochondrial retroplasmids routinely senesce at elevated temperatures, a process that is initiated by the integration of variant forms of the plasmids into the mitochondrial genome. Here, we describe a strain that is phenotypically distinguishable from previously characterized senescent strains and show that senescence can occur in the absence of plasmid integration and associated alterations in mitochondrial DNA. The MS4416 strain contains a unique variant of the Mauriceville retroplasmid, and undergoes senescence at highly predictable frequencies at 37°, 25° and 18 °C. Decline in vegetative growth rate correlates with increased levels of the variant plasmid and alterations in the synthesis of mitochondrially encoded proteins, suggesting that plasmid over-replication interferes with mitochondrial translation. We also report the isolation of a mutant strain that escapes senescence yet still maintains high levels of the variant plasmid. Its ability to tolerate a growth-suppressive retroplasmid suggests that there are mechanisms in Neurospora which compensate for the deleterious effects that plasmid over-replication has on mitochondrial function. Received: 12 July 1999 / Accepted: 17 December 1999     

tRNA-like structures of plant viral RNAs: conformational requirements for adenylation and aminoacylation

Bromo- and cucumovirus RNAs contain a tRNA-like structure as an integral part of their genome. This structure is located at the 3' end of the viral RNA and is an acceptor of tyrosine. The 3' regions of representative viral RNAs have been sequenced and quite unorthodox secondary foldings have been proposed for these 3' ends. The question therefore remained as to how these structures could be recognized by tRNA-specific enzymes. We have established the minimum number of nucleotides from the 3' end of the brome mosaic virus and broad bean mottle virus RNAs required for the formation of structures recognized by the tyrosyl-tRNA synthetase and/or the tRNA nucleotidyltransferase. The results obtained delineate the length of the tRNA-like region, and indicate that the 5' region of the tRNA-like structure participates in the formation of the amino acid stem. This has led us to propose an 'L'-shaped secondary structure for these tRNA-like regions.