Retroviral-type zinc fingers and glycine-rich repeats in a protein encoded by CAT2.urule;6, a Tetrahymena gene active during meiosis

Abstract

We have determined the nucleotide sequence of the cnjB gene from the ciliate Tetrahymena thermophila. This gene is transcriptionally active only during early conjugation, peaking in meiotic prophase. It contains 13 introns, four transcription start points and codes for a putative polypeptide (CnjB) of 1748 amino acids with a calculated molecular weight of 200 kilodaltons and a pl of 7.9. The coding region of cnjB has a low GC content (32% GC) and unusual codon usage. The C-terminal one-third of CnjB consists of three repetitive domains. Introns were absent in this region of cnjB. One of the repetitive domains consists of seven CCHC or retroviral-type zinc fingers, a motif found in one or two copies in retroviral nucleocapsid proteins. This motif has also been found recently in seven copies in the human nucleic-acid binding protein CNBP, in an apparent CNBP homologue in Schizosaccharomyces pombe and in one copy in a Xenopus gene active in early embryos. The other two domains are on either side of the zinc finger domain and contain a repeated glycine-rich motif seen in the heterogeneous nuclear ribonuclear proteins A1 and A2/B1 as well as other proteins. Both CCHC zinc fingers and glycine-rich repeats have been found in proteins with single-stranded nucleic acid-binding activity as well as strand-annealing activity. CnjB is, to our knowledge, the first protein found to contain both types of motifs.

Two GW Repeat Proteins Interact with Tetrahymena thermophila Argonaute and Promote Genome Rearrangement

“…The C-terminal portion of CnjBp contains two regions that are similar to the GW repeat domain of Wag1p by sequence but shorter in length. Also, there are seven CCHC zinc fingers (46) located between the two GW-rich regions. The C-terminal CnjBp region, which comprises about a third of the protein sequence, is predicted to be unstructured (see Fig.…”

The Conjugation-Specific Die5 Protein Is Required for Development of the Somatic Nucleus in both Paramecium and Tetrahymena

“…The N-terminal domains contain repeated elements with similarity to RNA binding motifs in other species, and evidence of nucleic acid binding activity was obtained. While there are Tetrahymena proteins, including CnjBp (41,51), that share similar glycine-rich repeats, none are clearly identifiable as homologues. Whereas the Pdd, Lia, and Nowa proteins reinforce the divergence between the DNA rearrangement machinery in the two species, Die5p reveals a novel connection between the two systems.…”

Reiterated WG/GW motifs form functionally and evolutionarily conserved ARGONAUTE-binding platforms in RNAi-related components

“…No clear role has yet been described for the T. thermophila conjugation-specific cnjB protein (Taylor and Martindale 1993), although RNAi has since been implicated in conjugation in this organism (Yao et al 2003). Interestingly, the C-terminal repeat region of TtcnjB shows an amino acid compositional bias similar to that of NRPD1b, being rich in tryptophan, glycine, serine, and asparagine residues.…”