Ribosomal proteins of the dimorphic fungus, Mucor racemosus

Mucor dimorphism

Mucor dimorphism has interested microbiologists since the time of Pasteur. When deprived of oxygen, these fungi grow as spherical, multipolar budding yeasts. In the presence of oxygen, they propagate as branching coenocytic hyphae. The ease with which these morphologies can be manipulated in the laboratory, the diverse array of morphopoietic agents available, and the alternative developmental fates that can be elicited from a single cell type (the sporangiospore) make Mucor spp. a highly propitious system in which to study eukaryotic cellular morphogenesis. The composition and organization of the cell wall differ greatly in Mucor yeasts and hyphae. The deposition of new wall polymers is isodiametric in yeasts and apically polarized in hyphae. Current research has focused on the identity and control of enzymes participating in wall synthesis. An understanding of how the chitosome interacts with appropriate effectors, specific enzymes, and the plasma membrane to assemble chitin-chitosan microfibrils and to deposit them at the proper sites on the cell exterior will be critical to elucidating dimorphism. Several biochemical and physiological parameters have been reported to fluctuate in a manner that correlates with Mucor morphogenesis. The literature describing these has been reviewed critically with the intent of distinguishing between causal and casual connections. The advancement of molecular genetics has afforded powerful new tools that researchers have begun to exploit in the study of Mucor dimorphism. Several genes, some encoding products known to correlate with development in Mucor spp. or other fungi, have been cloned, sequenced, and examined for transcriptional activity during morphogenesis. Most have appeared in multiple copies displaying independent transcriptional control. Selective translation of stored mRNA molecules occurs during sporangiospore germination. Many other correlates of Mucor morphogenesis, presently described but not yet explained, should prove amenable to analysis by the emerging molecular technology.

Role of lysine methylation in the activities of elongation factor 1 alpha

Previous work in our laboratory has demonstrated that 19% of the lysine residues in the protein synthesis elongation factor (EF-1 alpha) are methylated when the factor is purified from the mycelial form of the fungus Mucor racemosus. However, the same factor, when purified from spores of M. racemosus, is largely unmethylated. Despite its wide-spread occurrence in a great number of basic proteins, the functional significance of lysine N-methylation remains poorly understood. Spore and mycelial forms of EF-1 alpha were therefore compared in a series of assays to determine their relative affinities for various substrates and cofactors known to interact with the factor during the elongation cycle. The results suggested that hypomethylated and fully methylated EF-1 alpha had equal affinities for GTP, aminoacyl-tRNA, and ribosomes. Also, methylation did not appear to affect the accuracy of translation in an in vitro system. However, experiments did suggest that methylation may affect the ability of the factor to form complexes with other subunits (EF-1 beta gamma) which are known to enhance the overall rate of protein synthesis.

Structure of a ribosomal protein gene in Mucor racemosus

As an extension of our analysis of the translational apparatus of Mucor racemosus we have isolated a gene encoding a ribosomal protein of Mucor. Based on a method developed for S. cerevisiae, we identified by hybrid selection and in vitro translation a lambda-Charon 4A clone containing the genomic copy of a Mucor ribosomal protein. The gene consisted of two exons of 57 and 387 nucleotides. The two exons were separated by an 131 nucleotide intron. The processed transcript was 714 nucleotides in length and contained a 25 nucleotide untranscribed leader and an 114 nucleotide untranscribed 3'-end. The protein predicted from the nucleotide sequence contained 148 amino acids and exhibited 61% identity with the S19 ribosomal protein of Xenopus laevis. The promoter region of the gene contained sequences highly homologous to the RPG and Homol1 promoter elements found in S. cerevisiae. Southern blot analysis indicated that the Mucor genome contains three copies of this gene.

Sequence analysis of the EF-1 alpha gene family of Mucor racemosus

Our previous studies have shown that Mucor racemosus possesses three genes (TEF-1, -2 and -3) for EF-1 alpha, and that all three genes are transcribed. However, the level of transcription varies markedly between the three genes, with TEF-1 mRNA levels being approximately two fold higher than TEF-3 and 6 fold higher than TEF-2. We have now completed the DNA sequence of both strands of all three genes and have found that these genes are highly homologous. TEF-2 and TEF-3 are more similar to each other than they are to TEF-1. The TEF-2 and the TEF-3 coding regions differ from TEF-1 at 30 and 37 positions respectively out of 1374 nucleotides. Twenty-six of these nucleotide substitutions were common to both TEF-2 and TEF-3, and the majority of the substitutions were clustered in the 5' region of the coding sequences. While the majority of these changes were silent, TEF-2 and TEF-3 differed from TEF-1 by having a lysine instead of a glutamate at amino acid position 41. In addition, TEF-2 and -3, but not TEF-1, each have an intron located near the 5' end of the coding region, although its size and sequence is not conserved between the two genes. All three genes have a conserved intron near the 3' end of the coding region. The sequence data have been analyzed with respect to the structure and function of EF-1 alpha in protein biosynthesis.

Regulation of protein synthesis factor EF-1 alpha in Mucor racemosus

The protein synthesis elongation factor EF-1 alpha of Mucor racemosus hyphae contained eight or nine methylated amino acids per molecule, whereas the factor from sporangiospores was nonmethylated. During the course of spore germination, the specific activity of the factor in crude extracts increased sixfold. This increase in activity was accompanied by a constant level of EF-1 alpha-specific mRNA and a constant level of EF-1 alpha protein. Methylation of the protein, however, accelerated during the germination process, in parallel with the increase in specific activity of the factor. We propose that the activity of EF-1 alpha is regulated during germination through methylation of the protein and does not involve transcriptional regulation.

Regulation of protein synthesis factor EF-1 alpha in Mucor racemosus

The protein synthesis elongation factor EF-1 alpha of Mucor racemosus hyphae contained eight or nine methylated amino acids per molecule, whereas the factor from sporangiospores was nonmethylated. During the course of spore germination, the specific activity of the factor in crude extracts increased sixfold. This increase in activity was accompanied by a constant level of EF-1 alpha-specific mRNA and a constant level of EF-1 alpha protein. Methylation of the protein, however, accelerated during the germination process, in parallel with the increase in specific activity of the factor. We propose that the activity of EF-1 alpha is regulated during germination through methylation of the protein and does not involve transcriptional regulation.

Methylation of elongation factor 1 alpha from the fungus Mucor

A basic protein from the dimorphic fungus Mucor racemosus, found to be highly methylated, is shown to be protein synthesis elongation factor 1 alpha. This protein is the most abundant protein in hyphal cells but is less abundant in yeast cells. It is post-translationally methylated with the formation of mono-, di-, and trimethyllysine at as many as 16 sites. Nearly 20% of the 44 lysine residues of elongation factor 1 alpha from mycelia are modified while those from sporangiospores are virtually unmethylated.

Phosphorylation of ribosomal proteins S3, L1 and L24 during spherulation in Physarum polycephalum

The phosphate content of ribosomal proteins S3, L1 and L24 has been determined in the course of spherulation of Physarum polycephalum. The major phosphoprotein, S3, was completely dephosphorylated after 4 h of differentiation. The phosphate content of L1 and L24 was not altered during the differentiation. The cellular level of ATP remained constant for at least 5 h. A 3-fold reduction of cyclic AMP concentration occurred in the first hour, followed by a slow increase to a final value of twice the level observed in growing cells. The results showed that the phosphorylation of ribosomal proteins is regulated by at least two different mechanisms and that the dephosphorylation of S3 is not induced by a lack of cellular ATP. Although cyclic AMP might trigger the dephosphorylation of S3, the phosphate content of this protein remained at a very low value even when the cellular concentration of cyclic AMP rose significantly. Since the polysome level remains constant during the first 24 h of spherulation, the phosphorylation of S3 is not necessary for active protein synthesis and the phosphorylation of L1 and L24 is not involved in ribosome inactivation, which occurs after 24 h.

Turnover rates of phosphoryl groups in ribosomal proteins of Physarum polycephalum. Evidence for two different mechanisms

The rate of phosphate exchange in individual ribosomal proteins of Physarum polycephalum was determined in vivo. It was observed that the phosphoryl groups of S3, the major phosphoprotein, had a turnover rate of 1.5% per minute. The phosphoryl groups of proteins L1, L20 and L24 were stable. These results show that the phosphorylation of ribosomal proteins is regulated by at least two different mechanisms. The rapid turnover of phosphoryl groups of the major phosphoprotein is in agreement with the general observation that the phosphate content of this protein is modulated by the physiological state of the cells and possibly involved in the regulation of ribosome activity. The absence of phosphate exchange in acidic proteins suggests that these groups could play a structural role in the ribosome functions.

Relationship of internal cyclic AMP levels, rates of protein synthesis and mucor dimorphism

Changes in the intracellular pools of cyclic AMP and specific rates of protein synthesis have been described as correlates of the yeast-to-hypha conversion in Mucor racemosis. A further examination of the relationship between these physiological parameters and the cellular morphogenesis was conducted in the present study. The levels of intracellular cyclic AMP consistently varied as a function of the cell morphology rather than the CO2 tension, oxygen tension or growth rate. The specific rate of protein synthesis failed to change during a N2-to-air-induced yeast-to-hypha transition. Previously reported changes in this parameter during CO2-to-air- and CO2-to-N2-induced yeast-to-hypha shifts may be a consequence of growth rate changes rather than development per se.

Physiological control of phosphorylation ribosomal protein S6 in Mucor racemosus

The level of phosphorylation of ribosomal protein S6 increased with accelerating rates of growth and protein synthesis in Mucor racemosus. Lowered levels of phosphorylation were seen under conditions of metabolic shift-down or the onset of stationary phase, and no phosphorylation was detected in sporangiospores. Changing metabolic states, changing intracellular levels of adenosine triphosphatase, and the level of phosphorylation of protein S6 were correlated in M. racemosus.