Different structure of polytene chromosome of Phaseolus coccineus suspensors during early embryogenesis. 3. Chromosome pair VI

Different regions of polytene chromosomes pair VI have been characterized by: 1. morphological observations, 2. incorporation of 3H-thymidine and 3H-uridine, 3. cytophotometry of DNA and associated proteins, 4. hybridization with satellite DNA and highly repeated DNA sequences. The collected data indicate that DNA and RNA puffs are organized by heterochromatic segments. DNA puffs show often a clustered pattern of labeling by 3H-thymidine and RNA puffs are always labeled by 3H-uridine. Each heterochromatic segment is characterized by a definite ratio between DNA and associated fast-green stainable proteins. Satellite DNA binds mostly to heterochromatic blocks at centromere, highly repeated DNA sequences bind, with approximately the same frequency, to centromeric heterochromatin and to the main intercalary heterochromatic band. The telomeric portions of euchromatin seem to be also enriched in highly repeated DNA sequences. The results indicate that heterochromatic chromosome segments might be sites of intense localized DNA replication. The same chromosome regions are also engaged in an active transcription process. The response to hybridization suggests that heterochromatic blocks of chromosome pair VI are heterogeneous in nucleotide sequences. The present studies also indicate that DNA and RNA puffs organized by different chromosome sites are specific of particular steps of embryo differentiation. The observed metabolic aspects of the suspensors's polytene chromosomes are discussed in relation to the synthesis of growth regulators which is known to occur in the suspensor.