Removal of a telomere from yeast chromosome VII in a strain having two copies of this chromosome often results in its loss. Here we show that there are three pathways that can stabilize this broken chromosome: homologous recombination, nonhomologous end joining, and de novo telomere addition. Both in a wild-type and a recombination deficient rad52 strain, most stabilization events were due to homologous recombination, whereas nonhomologous end joining was exceptionally rare. De novo telomere addition was relatively rare, stabilizing <0.1% of broken chromosomes. Telomere addition took place at a very limited number of sites on chromosome VII, most occurring close to a 35-base pair stretch of telomere-like DNA that is normally approximately 50 kb from the left telomere of chromosome VII. In the absence of the Pif1p DNA helicase, telomere addition events were much more frequent and were not concentrated near the 35-base pair tract of telomere-like DNA. We propose that internal tracts of telomere-like sequence recruit telomerase by binding its anchor site and that Pif1p inhibits telomerase by dissociating DNA primer-telomerase RNA interactions. These data also show that telomeric DNA is essential for the stable maintenance of linear chromosomes in yeast.

The rpoB gene encoding the β‐subunit of RNA Polymerase has been isolated from the chloroplast genome of the chromophytic alga Heterosigma carterae (Taylor 1992). The nucleotide sequence contains an open reading frame of 3348 bp, which encodes 1116 amino acids. The H. carterae rpoB gene structure is similar to that of chlorophytic chloroplast rpoB genes with respect to insertion‐deletion domains. However, several conserved residues found in all chlorophytic plants and bacterial rpoB genes have not been found in this alga. This is the first nonchlorophytic chloroplast rpoB gene to be analyzed, and our data provide insight concerning regions, and perhaps particular residues, that may be essential to chloroplast RNA polymerase function.