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In several cell types, messenger RNA levels of the nuclear proto-oncogene c-myb vary as a function of cellular proliferation ; a transient increase in c-myb steady-state mRNA, mediated by post-transcriptional mechanisms, occurs during cell-cycle progression. In contrast, both quiescent and proliferating immature thymocytes contain exceptionally high levels of c-myb mRNA as a consequence of increased c-myb transcription.

In somatic cells the expression of replication-dependent histone genes is coupled to the S phase of the cell cycle. However, we have found a number of novel H2a, H2b, and H3 poly(A)+ RNA species in avian haploid round spermatids. The spermatid-specific H2a and H2b 0.8-kilobase RNAs are transcribed from a subset of the replication-dependent H2a and H2b gene families. Two cDNAs derived from the spermatid-specific H2b transcripts were isolated and sequenced. The structures of these cDNAs reveal that the spermatid-specific RNAs are identical to the 0.5-kilobase poly(A)- H2b mRNAs expressed in proliferating somatic cells, except for the addition of poly(A) at the 3' ends. The site of poly(A) addition in the spermatid-specific RNAs is located 26 to 28 nucleotides 3' of the poly(A)- H2b mRNA terminus. Thus, the hairpin structures and purine-rich elements required for the U7 small nuclear ribonucleoprotein-mediated cleavage reaction that generates the 3' ends of poly(A)- H2b mRNAs are not utilized in spermatids and are retained in the poly(A)+ H2b RNAs.

During mammalian spermiogenesis transitional proteins and protamines replace histones on the DNA as the chromatin condenses. While previous studies suggested that histone genes are inactive postmeiotically, we have shown both by steady-state RNA analysis and nuclear run-off transcription assays that histone 2b (H2b) transcription occurs in mouse round spermatids. In addition, a novel H2b cDNA clone has been isolated from an adult mouse testes cDNA library. The sequence of this cDNA clone predicts a protein that is extremely similar to other mouse H2b proteins, except at the carboxyl-terminus where the testes H2b contains an additional 12 amino acids, seven of which are hydrophobic. In contrast to the replication-dependent histone mRNAs, the 3′ untranslated region of this cDNA contains the poly(A) addition sequence (AAUAAA) upstream of a poly(A) tract. Furthermore, the conserved hairpin structure immediately upstream of replication-dependent histone mRNA termini is not present. Northern blot analysis of RNA from embryonic, ovarian, spermatogenic, and a variety of somatic tissues reveals that this novel H2b transcript is spermatid specific. The H2b mRNA is in polyribosomes isolated from spermatogenic cells, strongly suggesting that it is translated during spermiogenesis.

We sequenced and compared the telomeric regions of linear rDNAs from vegetative macronuclei of several ciliates in the suborder Tetrahymenina. All telomeres consisted of tandemly repeated C4A2 sequences, including the 5' telomere of the 11 kb rDNA from developing macronuclei of Tetrahymena thermophila. Our sequence of the 11 kb 5' telomeric region shows that each one of a previously described pair of inverted repeats flanking the micronuclear rDNA (Yao et al., Mol. Cell. Biol. 5: 1260-1267, 1985) is 29 bp away from the positions to which telomeric C4A2 repeats are joined to the ends of excised 11 kb rDNA. In general we found that the macronuclear rDNA sequences adjacent to C4A2 repeats are not highly conserved. However, in the non-palindromic rDNA of Glaucoma, we identified a single copy of a conserved sequence, repeated in inverted orientation in Tetrahymena spp., which all form palindromic rDNAs. We propose that this sequence is required for a step in rDNA excision common to both Tetrahymena and Glaucoma.

A simple repetitive DNA sequence in the nuclear genome of Trypanosoma brucei, consisting of tandem repeats of the hexanucleotide 5′ CCCTAA 3′, was identified as being telomeric by several criteria. This sequence was specfically labeled with T. brucei genomic DNA as the template for in vitro nick translation by DNA polymerase I, and was present in Bal 31 nuclease sensitive, genomic restriction fragments of the large sizes expected in this organism for at least some telomeric regions. The same repeated sequence was found in six other flagellates tested. A segment of DNA from T. brucei including this telomeric sequence was cloned in pBR322 and characterized. The cloned segment contained a sequence highly homologous to the 3′ ends of several variant surface glycoprotein mRNAs, upstream of the tandemly repeated hexanucleotide sequence.

The six chicken histone H1 genes have 3'-processing sequences typical of replication-dependent histone genes, which are expressed as poly(A)- mRNAs. However, by Northern analysis of RNA from several adult chicken tissues, as well as from embryonal skeletal muscle in vivo and in vitro, we have observed histone H1 transcripts longer than those predicted on the basis of the published genomic sequences. These RNAs are polyadenylated transcripts of the genes H1.01 and H1.10, which encode the 'c fraction' H1 protein subtypes. Both transcripts contain an internal stem-loop and purine-rich box associated with the 3' processing of poly(A)- histone mRNAs. The 2-kb poly(A)+ H1.01 transcript is present at high steady-state levels in tissues with low rates of DNA synthesis, has a longer half-life than the poly(A)- mRNA from the same gene, and is polyribosomal in embryonal skeletal muscle. The 1-kb poly(A)+ H1.10 RNA is the major H1.10 transcript in adult skeletal muscle. The properties of these RNAs suggest that they may contribute to the relaxed replication dependence of c fraction subtype expression. The polyadenylation signals of both genes are unusual in their association with processed (nonhistone) pseudogene-like elements, an arrangement with possible implications for the mechanism of alternative 3'-end formation in these genes.

The steady-state messenger RNA levels of several genes increase when cells are stimulated to proliferate. The transcripts from one such gene, the proto-oncogene c-myc, increase approximately 20-fold shortly after cells are stimulated to proliferate and then decline before the onset of DNA synthesis. It has been inferred from these data that expression of c-myc may be specific to the G1 portion of the cell cycle. Alternatively, this transient increase in c-myc mRNA following the stimulation of quiescent cells could be the result of an activational event that renders the cells competent to enter the cell cycle. To distinguish between these possibilities, we performed experiments to determine whether the amount of c-myc mRNA fluctuates during the cell cycle in cells that are under constant stimulation to proliferate. Although c-myc mRNA does undergo a transient increase within 2 h of serum stimulation of quiescent serum-deprived cells, our results show that the level of c-myc mRNA is constant throughout the cell cycle and does not diminish in density-arrested cells maintained in the presence of serum growth factors. In contrast to c-myc, the mRNA levels of two other genes whose expression has been associated with cellular proliferation do show consistent variations within the cell cycle. Both thymidine kinase (TK) and histone 2b (H2b) mRNA levels increase during S phase in continuously growing cells and decrease when cell replication ceases in density-arrested cultures. Therefore, the transient increase in c-myc transcription following the activation of quiescent cells is not due to the type of cell cycle-dependent regulation characteristic of the TK and H2b genes.

The bursa of Fabricius plays a central role in the development of B lymphocytes in avian species (Grossi 1976; Lydyard 1976; Ratcliffe 1985). The bursal anlage becomes populated with lymphoid stem cells between 8 to 14 days of embryogenesis. These cells begin to express histocompatibility class II antigens (Ia) from day 10 (Ewert 1984) and surface immunoglobulin (slgM) from day 12 of embryonic life (Grossi 1976). By the time of hatching at day 21 of embryogenesi s, nearly all (>95%) of bursal lymphocytes express the phenotype of a mature B cell (sIa+, sIgM+). After hatching, the bursa continues to increase in size as the number of cells in an individual follicle increases and B cells begin to seed to other organs. The bursa reaches its maximum size between 8 to 12 weeks of age and then begins to involute. By 6 months of age all that is left is a sclerotic remnant of the bursa. Several lines of evidence suggest that the bursa plays a central role in the development of B cells. Birds bursectomized at 17 days of embryogenesis fail to develop B cells and become completely agammaglobulinemic (Cooper 1969), suggesting that normally B cell differentiation occurs exclusively in the bursa. If, however, one ablates the bursal epithelium at 60 hours of incubation, the resulting chickens lack a bursa but do develop some circulating B cells and serum Ig (Jalkanen 1983).