TheAa mating locus is one of four multiallelic loci that govern sexual development in the basidiomycete fungus SchizophyUum commune. We have determined the nucleotide sequence encoding three Aa mating types, Aal, Aa3, and Aa4. We have found that the locus for Aa3 and Aa4 consists of two genes: Y and Z. The locus for Aal encodes only one gene, Y. The Z polypeptides encoded by different alleles exhibit 42% identity. The Y polypeptides exhibit 49-54% identity. The finding that the deduced Z and Y polypeptides have homeodomain motifs suggests that these polypeptides may be DNA-binding regulatory proteins that control the expression of developmental genes. The deduced Z polypeptide also has acidic regions that might be functionally analogous to the acidic regions in yeast GAL4 and GCN4 that activate transcription. The Y polypeptide has a serine-rich region and a basic region that shows some identity to the lysine-rich region of H1 histones.Control of mating between individual homokaryons of Schizophyllum commune is determined by four loci: Aa, A3, Ba, and B,8 (1,2). These loci regulate a developmental change from a nonfertile homokaryotic mycelium to a fertile dikaryotic mycelium capable of producing highly differentiated fruiting bodies. Meiosis and sporulation occur in these fruiting bodies. Two homokaryons must differ at Aa and/or At3 and differ at Ba and/or Bf3 to activate the entire sequence of events leading to the formation and maintenance of the dikaryon. The dikaryon is the product of a developmental sequence consisting of two different but complementary pathways; one is under control of the A loci; the other is under control of the B loci. Each of the loci Aa, Af3, Ba, and B,B has alternative forms, or mating types, that were identified by classical genetic analysis (3, 4). Nine Aa, 32 A/3, 9 Ba, and 9 B/3 mating types exist in nature. These mating types were envisioned as single genes and, therefore, were traditionally called alleles.The Aa loci from three mating types (Aal, Aa3, and Aa4) were recently isolated. Cross-hybridization experiments and limited DNA sequence data suggested that the three Aa sequences were quite heterogeneous (refs. 5-8 and C.A.S., unpublished data). The length of the heterogeneous region was estimated to be 5 kilobases (kb) for Aal and 8.5-9 kb for Aa3 and Aa4. To define further the structure ofthese loci, we have now determined the nucleotide sequence of major portions of the heterogeneous regions in Aal, Aa3, and Aa4. 11 This analysis has led to identification of two divergently transcribed multiallelic genes, designated Z and Y, at the Aa locus. The fact that these proteins contain a motif related to the homeodomain motif suggests that Z and Y are regulatory proteins. Specht et al. (9) provide a functional characterization of these genes. Together the analyses of Specht et al. (9) and this report provide insight into the structure of Aa and the mechanism by which Aa genes control development of the dikaryotic mycelium.MATERIALS AND METHODS Strains. Escherichia coli JM1...
The disappearance of F pili on Escherichia coli cells in the presence of 10-2 M NaCN was studied by electron microscopy and serum-blocking power. The pili which disappeared from the cell did not appear as free pili in the culture medium, suggesting that the pili had retracted into the cell. New pili were produced at a normal rate approximately 3 min after NaCN was removed. The adsorption of either F pili antibody or R17 bacteriophage to the sides of pili and temperatures below 24 C prevented retraction. The disappearance of pili was accompanied by a loss in the ability of cells to adsorb R17 phage and the type of F pili antibody that inhibits R17 phage infection and mating. The ability to adsorb M13 phage and the type of F pili antibody that inhibits M13 phage infection was not affected. This suggests that the tips of retracted pili are exposed.
The effect of temperature on the production of F pili by an F + strain of Escherichia coli B/r was studied by electron microscopy and by a technique involving serum-blocking power. The latter method is based on the ability of F pili to adsorb F pili antibody which inhibits male-specific phage infection. The total amount of pili in a sample was estimated by serum-blocking power; the length of F pili and number per cell was determined by electron microscopy. Cell extracts prepared by sonic oscillation lacked serum-blocking power, suggesting that F pili are not present in the cytoplasm. The number of F pili per cell varied with the growth temperature, but the average length of F pili remained constant. Maximum number of pili per cell occurs between 37 and 42 C; below 37 C the number decreases, reaching zero at about 25 C. When cells are grown at 37 C, blended, and resuspended in fresh media at 25 C, they make F pili. These pili are probably assembled from a pool of subunits that were synthesized during growth at 37 C. The rates of assembly at 25 and 37 C, as judged by the rate of increase in length of F pili, are similar. When cells were grown at 25 C and shifted up to 37 C, there was a 30-min lag in pili production followed by a period of rapid outgrowth. When cells were shifted down from 37 to 20 C, outgrowth (assembly) of pili ceased, and approximately 50% of the attached pili were released in 2 min. No release was observed when cells were shifted to 0 C. This suggests that pili may be released from the cell by a mechanism that requires metabolic activity, but not the outgrowth of F pili.
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