Nucleotide sequences are presented for 12, 7 and 12 cloned extrachromosomal DNAs by nature harbored in nucleoprotein (NP) complexes forming chicken leukaemic myeloblast (CHLM) post microsomal sediment (POMS) components A, B and C, respectively, and for 11 cloned avian myeloblastosis virus (AMV) DNAs. Analysis of the abundance of sequence motifs significant for eukaryotic chromosomal DNA replication origin (ori) regions (and their initiation zones) has shown that these DNAs are reminiscent of cell DNA fragments enriched in ori sequences (Rao et al., 1990) and/or sequence features of several eukaryotic chromosomal oris containing clusters of modular sequence elements (Dobbs et al., 1994). Accordingly, these DNAs, with an (A + T) content prevalently higher than that of the total cell DNA, revealed the presence of asymmetrically distributed (A + T)-rich stretches, scaffold attachment region (SAR) T consensuses, polypyrimidine nucleotide (poly(Py)) tracts and minimal Saccharomyces cerevisiae autonomously replicating sequence (ARS) consensus, in abundance comparable with that of these sequences of DNA fragments enriched in oris. All these DNAs were found to be enriched also in sequence elements held as primase (Pr) attachment sites. Moreover, DNAs of POMS component B and those of AMV DNA were found to be enriched in the asymmetric pyrimidine (Py) heptanucleotide motif of Waltz et al. (1996) occurring in the initiation zones of ori region. Consequently, these extrachromosomal DNAs, portion of which represents a precursor of AMV DNA, seem to descent from initiation zones of various ori regions of an early replicating chromosomal myeloblast DNA. In addition, a possible explanation of the inclination of these DNAs to form multimers is presented.

Institute of Molecular Genetics Academy of Sciences of Czech Republic Prague Czech Republic