Cyclin-dependent kinases (Cdks) control cytoskeleton polarization in yeast morphogenesis. However, the target and mechanism remain unclear. Here, we show that the Candida albicans Cdk Cdc28, through temporally controlled association with two cyclins Ccn1 and Hgc1, rapidly establishes and persistently maintains phosphorylation of the septin cytoskeleton protein Cdc11 for hyphal development. Upon hyphal induction, Cdc28-Ccn1 binds to septin complexes and phosphorylates Cdc11 on Ser394, a nonconsensus Cdk target. This phosphorylation requires prior phosphorylation on Ser395 by the septin-associated kinase Gin4. Mutating Ser394 or Ser395 blocked Cdc11 phosphorylation on Ser394 and impaired hyphal morphogenesis. Reconstitution experiments using purified Cdc28-Ccn1, Gin4, and septins reproduced phosphorylations on the same residues. Transient septin-Cdc28 associations were also detected prior to bud and mating-projection emergence in S. cerevisiae. Our study uncovers a direct link between the cell-cycle engine and the septin cytoskeleton that may be part of a conserved mechanism underlying polarized morphogenesis.

We describe here the presence of two distinct types of rRNA operons in the genome of a thermophilic actinomycete Thermomonospora chromogena. The genome of T. chromogena contains six rRNA operons (rrn), of which four complete and two incomplete ones were cloned and sequenced. Comparative analysis revealed that the operon rrnB exhibits high levels of sequence variations to the other five nearly identical ones throughout the entire length of the operon. The coding sequences for the 16S and 23S rRNA genes differ by approximately 6 and 10%, respectively, between the two types of operons. Normal functionality ofrrnB is concluded on the basis of the nonrandom distribution of nucleotide substitutions, the presence of compensating nucleotide covariations, the preservation of secondary and tertiary rRNA structures, and the detection of correctly processed rRNAs in the cell. Comparative sequence analysis also revealed a close evolutionary relationship between rrnB operon of T. chromogena and rrnA operon of another thermophilic actinomycete Thermobispora bispora. We propose thatT. chromogena acquired rrnB operon fromT. bispora or a related organism via horizontal gene transfer.

The lck gene encodes a lymphocyte-specific protein-tyrosine kinase that is implicated in T cell maturation and signaling. In mammals, the transcription of the lck gene is regulated by two independent promoters, the proximal promoter, which is active in thymocytes, and the distal promoter, which dominates in mature T cells. In the human and mouse lck gene loci, the two promoter elements are separated by at least 40 kb and 10 kb, respectively. In this study, we have cloned and sequenced 60 kb from the pufferfish (Fugu rubripes) lck locus. The promoter region of the Fugu lck spans only 4.2 kb and contains a proximal and a distal promoter in the 2.3-kb region adjacent to the coding sequence. By generating transgenic mice, we have demonstrated that the compact promoter of the Fugu lck contains regulatory elements that direct expression to lymphoid organs of mice. We were able to localize the regulatory elements to a short region of 830 bp without losing specificity to cultured human T cell line. These results show that the basic mechanisms that mediate lymphocyte-specific expression are conserved between teleosts and mammals. The short promoter of the Fugu lck isolated by us offers a powerful tool for labeling T cells, targeting expression, and manipulating T cell activity in fishes as well as in mammals.T he specificity of gene expression for cell types or developmental stages and the response of genes to physiological stimuli are mediated through a combinatorial interaction of promoter sequences, enhancers, and suppressors. These regulatory elements are composed of short stretches of DNA that are generally found in the promoter region of genes but also can be located in introns or dispersed over many kilobases upstream and downstream of genes (1). Finding these short elements in mammalian genomes is a formidable task because the intergenic regions in these genomes are large and complex. The genome of the pufferfish, Fugu rubripes, which is approximately eight times smaller than the human and mouse genomes, contains compact intergenic and intronic regions that are devoid of repetitive sequences (2, 3). This genome offers an attractive model for the rapid screening of noncoding sequences for conserved putative regulatory elements. Conserved regulatory elements driving cell-and stage-specific expression of developmental control genes such as the Hoxb-4, Otx2, Wnt-1, and Pax6 were identified by this strategy (4-7). Conserved regulatory elements were identified in the Fugu neurohypophyseal gene that express specifically in a subset of neurons in the hypothalamus of transgenic rats (8), and the Fugu tyrosinase gene was shown to contain conserved elements that directed melanocyte and retinal pigment epithelium-specific expression in transgenic mice (9). The hypothesis can be proposed that, where physiological systems show conservation over long stretches of evolutionary time, the genes specifying these processes are likely to be conserved not only in their coding sequences but also in their regulatory sequences. The immun...

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