Early replication and expression of oocyte-type 5S RNA genes in a Xenopus somatic cell line carrying a translocation.

Abstract

In Xenopus somatic cells, the somatic-type 5S RNA genes replicate early in S phase, bind the transcription factor TFIIIA, and are expressed; in contrast, the late replicating oocyte-type genes do not bind TFIIIA and are transcriptionally inactive. These facts support a model in which the order of replication of the somatic-type versus the oocyte-type SS genes causes their differential expression in somatic cells due to sequestration of TFIIIA by the early-replicating somatic genes. Here we provide further evidence for the model by showing that in one Xenopus cell line in which some oocyte-type 5S genes are translocated, some oocyte-type 5S genes replicate early and are expressed.In eukaryotes, chromosomal regions are replicated at different times in S phase (1, 2). Funthermore, particular regions appear to be replicated at the same time during successive S phases (3 In Xenopus laevis there are two multigene families of 5S RNA genes (8): the oocyte-type 5S genes, which are expressed only in oocytes (9, 10), and the somatic-type 5S genes, which are expressed throughout development (10). There are 20,000 copies of the oocyte-type 5S genes and 400 copies of the somatic-type 5S genes per haploid genome (11), all of which are located at the distal ends of the long arms of most Xenopus chromosomes (12, 13). We found that, in somatic cells, the somatic-type genes replicate early in S phase, whereas the oocyte-type genes replicate late (14). These data show a correlation between early replication and expression for genes transcribed by RNA polymerase III that is consistent with the work noted above on genes transcribed by RNA polymerase II. The findings also provide support for the replication-expression model proposed by Gottesfeld and Bloomer (15) One prediction of the replication-expression model is that an alteration in the timing of replication of5S genes should be accompanied by an alteration in their expression. We exploited a Xenopus somatic cell line in which some of the 5S genes are translocated to a pericentric position to investigate the relationship among chromosomal position, time of replication, and expression. Pardue et al. (12) had previously shown by in situ hybridization that, in addition to typical telomeric clustering of 5S genes, this line possesses a chromosome with two 5S clusters, one telomeric and the other pericentromeric. However, they did not determine which type of 5S gene was translocated to the pericentromeric location. Interestingly, Ford and Mathieson (19) showed by fingerprint analysis that there is some oocyte-type 5S RNA synthesized in this cell line. The cell line, originally established by K. A. Rafferty in 1968, has not been named. Based on the work reported here, we refer to it as TrXo (Translocated Xenopus oocyte-type 5S genes). In the present study, we show that the translocated 5S RNA genes in TrXo are oocyte-type. In addition, the expression of some oocyte-type 5S genes in this cell line was confirmed by a new assay. Finally, we determined that in TrXo some oocyte-typ...

Silk gland-specific tRNA(Ala) genes are tightly clustered in the silkworm genome.

“…Evidence consistent with this replication-expression model has been found in a Xenopus tissue culture cell line in which approximately 5% of the oocyte-specific 5S RNA genes have undergone a translocation that places them near the centromere (4,27). In this cell line, approximately 10% of the total oocyte-specific 5S RNA genes are replicated early and low levels of oocytespecific 5S RNA accumulation are seen, whereas in cell lines that do not carry the translocation, all of the oocyte-specific 5S RNA genes are replicated late and no expression of them is detectable (11). However, since it is not known whether it is actually the translocated oocyte-specific 5S genes that are expressed in the novel cell line, the connection between Other models that link tissue-specific expression with genomic organization, particularly clustering of genes, invoke a more direct connection between physical arrangement and accessibility to the transcription apparatus.…”

Long-Distance Control of Origin Choice and Replication Timing in the Human β-Globin Locus Are Independent of the Locus Control Region

“…Constitutively active housekeeping genes, for example, are typically early replicating, while most tissue-specific loci are early replicating in cells in which they are expressed and late replicating in cells in which they are not (26; reviewed in references 28 and 48). In addition, significant changes in both replication timing and transcriptional activity occur during X chromosome inactivation in mammals (54,56), immunoglobulin heavy-chain rearrangement in B cells (6,8), and in chromosomal translocations, including those that lead to human leukemias (14,24,34). Despite these possibilities, our results demonstrate that the transcriptionally inactive ⌬HS2-5 locus in MEL and GM979 erythroid cells is early replicating.…”

Specific regulation of Xenopus chromosomal 5S rRNA gene transcription in vivo by histone H1.

“…Plasmids containing either the X. laevis major oocyte 5S rRNA gene (pXP5Soo) or the somatic 5S rRNA gene {pSP64-Xls11; Guinta et al 1986) were linearized with HindlII and BamHI, respectively. Unlabeled antisense RNA was generated using either SP6 RNA polymerase (pXP64-X1 s 11) or T7 RNA polymerase (pSPSSoo).…”

“…The reaction was stopped by precipitation with ethanol. RNAs were resolved on a 6% polyacrylamide gel {see Guinta et al 1986;Andrews et al 1991).…”