SKU5-Similar 6 (SKS6) is a one of a large gene family of 19 members in Arabidopsis thaliana (L.) Heynh that encode multicopper oxidase-like proteins that are related to ferroxidases, ascorbate oxidases and laccases. Only one member of the family has been previously studied; Skewed5 (SKU5) is involved in the control of root growth. The encoded SKS6 protein, like SKU5 appears to lack a functional copper-binding site and is most closely related to Bp10 from Brassica napus and Ntp303 from Nicotiana tobacum. The SKS6 promoter contains many putative regulatory sites and differential expression of an SKS6::GUS reporter gene revealed selective induction in several seedling tissues including guard cells, root cortex cells, and leaf margin hydathodes. It was also expressed later in flower development in flower primordia, ovules, and the abscission zones of seeds and siliques. Furthermore, SKS6 was upregulated in roots in response to treatment of seedlings with the hormones abscisic acid, indole-3 acetic acid, 2,4-dichlorophenoxyacetic acid and aminocyclopropane-1-carboxylate. A loss-of function sks6-1 T-DNA insertion allele revealed that cotyledon vascular patterning is affected in the mutant, suggesting a role for the protein in metabolism of nutrients or hormones in the hydathodes, the sites of auxin synthesis and chemical recycling.

A protein kinase, specific for 60s ribosomal proteins, has been isolated from Saccharomyces cerevisiae cells, purified to almost homogeneity and characterized. The isolated enzyme is not related to other known protein kinases. Enzyme purification comprised three chromatography steps ; DEAEcellulose, phosphocellulose and heparin -Sepharose. SDSjPAGE analysis of the purified enzyme, indicated a molecular mass of around 71 kDa for the stained single protein band. The specific activity of the protein kinase was directed towards the 60s ribosomal proteins L44, L44, L45 and a 38 kDa protein. All the proteins are phosphorylated only at the serine residues. None of the 40s ribosomal proteins were phosphorylated in the presence of the kinase. For that reason we have named the enzyme the 60s kinase. An analysis of the phosphopeptide maps of acidic ribosomal proteins, phosphorylated at either the 60s kinase or casein kinase 11, showed almost identical patterns. Using the immunoblotting technique, the presence of the kinase has been detected in extracts obtained from intensively growing cells. These findings suggest an important role played by the 60s kinase in the regulation of ribosomal activity during protein synthesis. 20 years ago, when Kaltschmidt and Wittmann described the gel electrophoresis procedure for the separation and identification of Escherichia coli ribosomal proteins [l], the two acidic proteins L7 and L12 (12 kDa) of the 50s subunit have become one of the most studied ribosomal proteins with respect to their three-dimensional structure and biological function.The functional and physicochemical equivalents of the procaryotic L7 and L12 are two ribosomal proteins, L44 and L45, from the eucaryotic 60s subunit (for review see [2]). Contrary to in animals, the ribosomes of Saccharomyces cerevisiae cells contain a third low-molecular-mass acidic protein. That protein, named L44', can be separated on DEAEcellulose [3] and by an electrofocusing procedure in acrylamide gel [4]. All three proteins have molecular masses of about 13 kDa, belong to 'split proteins ' [5] The phosphorylation of the crude eucaryotic ribosomes was initially reported by Kabat [lo] and by Loeb et al. [ll]. Later studies by other authors have shown that mammalian ribosomes are associated with cyclic-AMP-independent protein kinases [12-151. One of the enzymes has turned out to be a casein kinase type I1 (CKII) which phosphorylates proteins L44 and L45 [14]. For the yeast ribosomes, it has been found that the majority of phosphoproteins, y-32P labelled in vivo [16 -191 correspond to the proteins phosphorylated in vitro with . Two of them were identified as L44 and L45 [18, 19, 211 and are phosphorylated in vitro by the casein kinase type I1 [21, 221. Studies on the physiological role of the phosphorylation of ribosomal acidic proteins have been in progress for 10 years [23] and involved the yeast system. As shown in Ballesta's laboratory, the phosphorylation of L44, L44 and L45 might control the binding process of those proteins to ribosom...