i : heterokaryon incompatibility-i
Used in early papers as the symbol for het-i. The symbol i was also used for en(am)-1 and for an unmapped intensifier of carotenoid pigment (1904).
Iasc : Indurated ascus
VR. Right of Cen-V (1968).
The wall of an indurated ascus hardens and darkens, making the entire ascus resemble a giant ascospore. Pores are formed and striations appear. The spore can be induced to germinate. Dominant with variable expression. Some heterozygous asci are normal. The ascus phenotype resembles that described for indurated asci of Neurospora tetrasperma (546). Vegetative growth is weak (1966). Indurated asci can be produced by biotin deficiency in wild-type strains of N. tetrasperma (725, 1680). The Neurospora crassa strain deposited in FGSC as Iasc (FGSC No. 3424) has been reported not to produce indurated asci, but the crosses were made on medium supplemented with biotin. Whether biotin concentration affects expression in this strain has not been determined. (In contrast to Iasc, the single giant ascospore in the Banana mutant is formed by hardening of the ascus membrane within a normal wall rather than by hardening of the ascus wall itself.)
ile-1 : isoleucine-l
VII. Between ars (l%) and wc-1 (3%). Probably right of met-7 (<1-2%) (1328, 1582, 1585).
Uses isoleucine, a-amino-n-butyric acid, threonine (2064), or canavanine (129). Affects threonine dehydratase (EC 4.2.1.16) (1056, 1059) (synonym; threonine deaminase; 1056) (Fig. 39). Leaky on minimal medium; treacherous to score with large inocula. Tests should be read early (24 hr if at 34ºC). Moderate inhibition by methionine (2064). May be screened as tiny germlings from ascospores germinated on minimal medium (1328). Name changed from thr-1 (1056).
ilv : isoleucine + valine
Mutants designated ilv require both amino acids. The three ilv loci specify enzymes that catalyze corresponding steps in the parallel biosynthetic pathways of isoleucine and valine (Fig. 39). These enzymes are located in the mitochondria (157, 1161), and they may indirectly affect electron transport (157). The enzymes may be in an aggregate; see review in ref. (456). A ratio of 20-30% isoleucine to 80-70% valine is optimal for growth (194). At least some ilv mutant strains are inhibited by norleucine, norvaline, phenylalanine (194), or tryptophan (1172).
Regulation: Enzyme production in response to end-product-derived signals depends on leu-3+ product and a-isopropylmalate. In leu-3+, threonine deaminase production is repressed as a function of available isoleucine, acetohydroxy acid synthetase as a function of valine, and isomeroreductase and dihydroxy acid dehydratase as a function of isoleucine and leucine. In the absence of effective leu-3 product and/or a-IPM, enzyme production is repressed even under severe end-product limitation (1497). Called iv.
FIGURE 39 The biosynthetic pathways of isoleucine, valine, and leucine, showing sites of gene action (37 , 290 , 798 , 1056 , 1419 , 1670 , 2174 ). Isoleucine and valine are synthesized along parallel pathways catalyzed by common enzymes. The leucine precursors Alpha- and Beta-isopropylmalate formerly were called Beta-hydroxy- Beta-carboxyisocaproate and Alpha-hydroxy- Beta-carboxyisocaproate. From ref. (1596 ), with permission from the American Society for Microbiology.
ilv-1 : isoleucine + valine-1
VR. Between per-1 (4%) and ilv-2 (<1%; 9%), lys-2 (4%; 7%) (10, 21, 919). Crosses with ilv-2 gave prototroph frequencies that were variable and sometimes unusually high (<1% to 9%) (1069).
Requires both isoleucine and valine or the corresponding keto acids (1069, 2138, 2174). Affects dihydroxy acid dehydratase (EC 4.2.1.9) (37, 1419) (Fig. 39). Most alleles are leaky (1069). Leucine has a sparing effect on the valine requirement (194). Called iv-1; groups 2 and 3.
ilv-2 : isoleucine + valine-2
VR. Between ilv-1 (<1%; 9%) and lys-2(1069, 1919).
Cloned and sequenced: Swissprot ILV5_NEUCR, EMBL/GenBank M84189, EMBL NCILV2A, GenBank NEUILV2A; pSV50 clones 30:5C, 30:10F.
Structural gene for a-keto-b-hydroxylacyl reductoisomerase (EC 1.1.1.86) (1919, 2174). Requires both isoleucine and valine (Fig. 39). Known alleles are not leaky (1069). Allele T313 is heat-sensitive (1069). The ILV-2 protein is present at high levels in the cell (1070). Called iv-2; group 1.
ilv-3 : isoleucine + valine-3
IVR. Linked to met-2 (0/129) (1582). Between leu-2 (4%) and ad-6 (9%) (985, 1124).
Cloned: pSV50 clone2:4C.
Requires both isoleucine and valine. Accumulates pyruvate. Very low acetohydroxy acid synthetase activity (EC 4.3.1.18) (290) (Fig. 39). Markedly inhibited by methionine (1582). The level of ilv-3 and leu-1 mRNAs increases in an ilv-3 mutant starved for branched-chain amino acids (985). Alleles recombine and complement (1124, 2174). Called iv-3; group 4.
In(. . . ): Inversion
A rearrangement in which two or more contiguous genes are inverted relative to the normal sequence. For information on published individual inversions, see ref. (1578).
In(OY323) : Inversion (IL;IR) (OY323)
A pericentric inversion with breakpoints between nit-2 and leu-3 in IL and between lys-3 and ace-3 in IR (115). Reference (1578) is incorrect in stating that the right breakpoint is left of lys-3.
inl : inositol
VR. Between pho-2 (3%; 4%), al-3 (1%; 1T/18 asci) and pab-1 (1%; 10%) (673, 747, 1447, 2014).
Cloned: pSV50 clones include 13:10B, 17:9D, 18:6C.
Lacks myoinositol-1-phosphate synthase (glucocycloaldalase, EC 5.5.1.4). Growth is colonial on low levels of inositol (698). Tends to extrude dark pigment into the medium when grown on suboptimal inositol. Composition of phospholipids and cell walls is abnormal on limiting inositol (698, 829, 830, 934) (Fig. 18). Inhibited by hexachlorocyclohexane (697, 877, 1827). Conidia are subject to death by unbalanced growth on minimal medium (2003, 2010), a property exploited for mutant enrichment (“inositolless death”) (1181, 1292) because double mutants are at a selective advantage. Heat-sensitive allele 8320, which grows at one-third the wild-type rate on race tubes, is especially useful for mutant enrichment (1631, 2022). Used in the first experiments reporting the transformation of Neurospora (1356, 1358, 2245). Used to study transport systems for glucose (1792) and sulfate (1280). Used for studying induced reversion (732). Used for studying the mechanism of inositol-less death (1292, 1391), the mutagenicity of ferrous ions, and regulation of mitochondrial membrane fluidity; reviewed in ref. (1391). Spontaneous reversion rates (727). Heat-sensitive allele 83201 shows slow semicolonial growth in liquid minimal at 25ºC (1280), but looks normal on slants (1546). opi acts as a partial suppressor of allele 89601, which contains cross-reacting material (2305). su(inl) is an allele-specific partial suppressor of another inl allele (735). A mutant exo-1 allele is present in inl (89601) stock FGSC 498 and may, therefore, be present in stocks of mutants derived from it by inositol-less death (372, 615, 2002). Allele 46802 is nonrevertable and inseparable from T(46802) (727, 1578). Called inos.
inos : inositol
Symbol changed to inl.
int : intense
IVR. Linked to pan-1 (0/50) (1592).
Brighter orange than wild type, perhaps because of morphology rather than carotenoid content (1592).
inv : invertase
VR. Between ro-4 (5%; 8%), pab-2 (3%), mus-11 (4%) and asn (4%; 9%) (1582, 1794, 1828).
Cloned: Plasmid pNC2 (293).
Structural gene for invertase (EC 3.2.1.26). Unable to use sucrose as the carbon source. Grows well on glucose or fructose and fairly well on casamino acids or yeast extract. Uninducible by normal inducers (1793). Mutants can be identified efficiently by a procedure using o-tolidine, glucose oxidase, and horseradish peroxidase to assay colonies in situ by halo formation (1150). Invertase also is affected by cot-2.
ipa : it pokes along
IL. Between mat (20%) and arg-1 (1%) (110, 1932).
Hyphae from germinating ascospores or conidia grow out for long distances without branching. Cultures are one day late growing up (110). Modifies pro-3. The double mutant pro-3; ipa does not respond to arginine and does not grow as well as pro-3; ipa+ on proline, citrulline, or ornithine. As a single mutant, ipa grows on minimal medium at half the wild-type rate. Arginine uptake is normal; arg-2; ipa or arg-5; ipa can grow on arginine. Inhibition studies suggest that ipa may be unable to shunt exogenous arginine into the proline pathway (1932).
ipm-1 : isopropylmalate-1
Unmapped. Unlinked to ipm-2 or leu-4 (1708).
Altered isopropylmalate (IPM) permeability. The mutant is able to use b-isopropylmalate to support the growth of leu-4 and for the induction of b-IPM isomerase and b-IPM dehydrogenase in contrast to ipm+, which is unable to take up this intermediate (1708, 1709).
ipm-2 : isopropylmalate-2
Unmapped. Unlinked to ipm-1 or leu-4 (1708).
Altered isopropylmalate permeability. Improves b-IMP uptake of ipm-1 mutants in supporting the growth of leu-4 (1708, 1709). The ipm-2 single mutant is relatively ineffective in promoting b-IPM permeability.
iv : isoleucine + valine
Symbol changed to ilv.
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