The complete nucleotide sequence of Saccharomyces cerevisiae chromosome X (745 442 bp) reveals a total of 379 open reading frames (ORFs), the coding region covering approximately 75% of the entire sequence. One hundred and eighteen ORFs (31%) correspond to genes previously identified in S. cerevisiae. All other ORFs represent novel putative yeast genes, whose function will have to be determined experimentally. However, 57 of the latter subset (another 15% of the total) encode proteins that show significant analogy to proteins of known function from yeast or other organisms. The remaining ORFs, exhibiting no significant similarity to any known sequence, amount to 54% of the total. General features of chromosome X are also reported, with emphasis on the nucleotide frequency distribution in the environment of the ATG and stop codons, the possible coding capacity of at least some of the small ORFs (<100 codons) and the significance of 46 non‐canonical or unpaired nucleotides in the stems of some of the 24 tRNA genes recognized on this chromosome.

The study of plant DNA polymerases lags far behind that concerning their animal or yeast counterpart. In this work we describe the first extensive purification to apparent homogeneity, as well as a detailed biochemical and immunological characterization, of a low molecular weight DNA polymerase (DNA polymerase CI) purified from wheat embryos. The monomeric enzyme is a basic protein having a molecular weight of 52 kDa. Polyclonal antibodies raised in rabbits against DNA polymerase CI did not inhibit animal DNA polymerases alpha and beta or wheat DNA polymerase A, whereas wheat DNA polymerases CII and B were much less affected than the CI enzyme. Several properties of enzyme CI were studied. Some known inhibitors of DNA polymerase activity including aphidicolin, phosphonoacetic acid and heparin, did not affect DNA polymerase CI while the activity of this enzyme was strongly inhibited by ddTTP and N-ethylmaleimide. The polyamine spermine decreased markedly the enzyme activity, while spermidine produced a strong stimulation at the same concentrations that spermine inhibited the enzyme. The best template for this enzyme is poly dA-oligo dT, although polymerase CI can recognize significantly some synthetic polyribonucleotide templates (poly rC-oligo dG, poly rA-oligo dT) but only at a given protein/template primer ratio. The enzyme is blocked at the amino terminus, thus preventing the automatic sequencing of the protein. The amino acid analysis showed a striking similarity with the animal low molecular weight DNA polymerase beta. The latter observation, as well as the effect of inhibitors (except N-ethylmaleimide which does not inhibit the animal polymerase) indicate that the DNA polymerase described in this work is a plant DNA polymerase very similar to the low molecular weight animal DNA polymerase beta, an enzyme believed to be involved in nuclear DNA repair.

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