The purpose of these studies was mainly the investigation of a variegated position effect affecting the conidiation system of Aspergillus nidulans. Genetical and biochemical investigations were undertaken to explore the nature of the variegated process and therefore gain information about the mechanism(s) underlying the relationship of chromatin structure and gene expression. The main lines towards which these studies were finally orientated include (a) studies of the brlA12 by means of modifiers isolated as such or found in stock strains; such studies aim to trace other features of these mutants and to correlate them to their modifying action; (b) studies of the brlA12 by means of environmental effects on its variegation, caused by chemicals administered in the growth medium; such studies would produce evidence about the biochemical nature of the variegation; (c) search for biochemical defects either in brlAl2 itself or its modifiers in combination with the other approaches. The main points which emerged from the above studies can be summarised here as follows; (1) brlA12 grows normally on any nitrogen or carbon sources tested. Its variegation is shifted towards wild type appearance (increased conidiation) by salts, methylamine, and to a minor extent, by a number of other compounds, low pH and temperature. (2) A number of mutants isolated as such or found in the stock strains act as suppressors or enhancers of the variegation (increasing or reducing the conidiation further, respectively). (3) One class of the suppressors, represented by rev-2 is unable to utilize galactose as carbon source; no clear pattern correlating galactose utilization and brlA12 variegation has emerged however. A connection of galactose utilization and polyamine metabolism may exist (as in yeasts) since iproniazid, an amine oxidase inhibitor, repairs to some extent growth of these suppressors on galactose. (4) The suppressors which are defective on galactose are extra sensitive to molybdate, but this pattern is followed only by modifiers isolated as such; gam and some gal mutants which are all defective on galactose and enhance the brlA12 mutant are molybdate resistant. (5) Molybdate resistant strains (isolated as such or found to be modifiers of brlA12) act as enhancers with the exception of mo1A which has no effect on the variegation; this mutant however may have a different basis of resistance. (6) rev-2 has low viability rate and high spontaneous mutation frequency to selenate resistance. (7) rev-2 has subnormal growth on putrescine as nitrogen source and it is pigmented pink. The pigmentation is greatly enhanced if nitrate (or nitrite) is present at the same time (glucose as carbon source), in which case conidiation is extremely poor. (8) brlA12 and to a lesser extent all the brl mutants (non-variegated alleles) are also pigmented on putrescine plus nitrate but not on putrescine alone. (9) Preliminary investigations suggest that the pink pigment, which may be relevant to conidiation processes, may be a prodigiosin-like pigment with putrescine or a compound derived from putrescine as its biosynthetic precursor. (10) rev-2, rev-5, rev-7 do not alter the conidiation pattern of the non-variegated leaky br1A9; this may indicate that these suppressors affect chromatin structure in general rather than being conidiation specific. (11) Studies of polyamine metabolism did not reveal any differences in terms of polyamine uptake or putrescine oxidase and transaminase activities. An instance of possibly abnormally high internal putrescine pools for rev-2 after growth on this amine needs further investigation. Although studies of amine metabolism in this work were orientated to their connection with brlA12, a number of general aspects of polyamine metabolism and regulation in Aspergillus nidulans have been investigated; a number of mutants altered in polyamine metabolism were isolated, two catabolic enzymes for putrescine were characterised, and the uptakes and internal polyamine pools were studied. Also an activation of glutamate dehydrogenase by polyamines was examined. (Abstract shortened by ProQuest.)
