Lampbrush chromosomes and gene utilization in meiotic prophase.

ABSTRACT

The main features of lampbrush chromosome organization are reviewed and the significance of RNA transcription on lampbrush loops is questioned. Special consideration is given to evidence for the transcription on lampbrush loops of satellite DNA, low copy number genes with defined functions, the histone genes, the 5S genes and the genes for ribosomal RNA. It is concluded that there is widespread but somewhat indiscriminate transcription on lampbrush loops of a range of repetitive DNA sequences, transcription of a wide range of mRNAs, continuous transcription of histone and 5S RNA, and low or aberrant transcription of ribosomal RNA. The 'read-through' hypothesis of lampbrush loop transcription is explained and evaluated, and some of the assumptions underlying it and the problems it raises are examined and discussed. The hypothesis requires that transcription starts at a normal promoter site for a functional gene situated at the thin end of a lampbrush loop, and that once started the transcribing polymerase cannot stop until it reaches another promoter that is already initiated or some condensed and untranscribable chromatin. The following questions are considered. Why should polymerase on a lampbrush loop disregard normal termination signals? Why should polymerase stop when it encounters another initiated promoter sequence? Why is the number of loops or transcription units related to genome size signifying, according to the read-through hypothesis, that oocytes from animals with large genomes have more active 'functional gene promoters' than those from animals with small genomes? Finally, some special situations where there is enhanced or reduced lampbrush activity are considered and their significance in relation to ideas about the function of lampbrush chromosomes is discussed. Specific examples include the frog, Ascaphus truei, whose oocytes have eight germinal vesicles each with a full complement of lampbrush chromosomes, another frog, Flectonotus pygmaeus, in which each oocyte starts with several thousand meiotic nuclei only some of which go into a lampbrush phase, and certain species of reptile in whose germinal vesicles the chromosomes never acquire a lampbrush form.