|Chromosome||genome: 0.81 - 0.81 Mb|
|Frequency clock protein|
Frequency (frq) is a gene discovered in the fungus Neurospora crassa in 1978 that encodes the protein frequency. The gene is 2,980bp long in Sordaria macrospora k-hell. The FRQ protein plays a key role in the autoregulatory transcription translation negative feedback loop (TTFL), which is responsible for circadian rhythms in N. crassa and other fungi such as N. sitophita, N. tetrasperma, N. galapagosensis, C. spinulosa, and L. australiensis.
Malcolm L. Sargent, Winslow R. Briggs and Dow O. Woodward at Stanford University discovered the presence of a CLOCK in Neurospora crassa in 1966. Jerry F. Feldman and Marian N. Hoyle discovered the frq mutant genes (frq-1, frq-2, and frq-3) in 1973. In 1986, frq was first cloned which allowed for further research and understanding of the CLOCK mechanism.
Frq’s important circadian function is supported by experiments showing that deletion of the frq gene results in arrhythmicity. Frq forms two transcripts that encode two different FRQ proteins, a long form of 989 amino acids (lFRQ) and a shorter form of 890 amino acids (sFRQ). Temperature determines the ratio at which both FRQ proteins are present, but both are required for strong rhythmicity. However, the CLOCK is able to persist at certain temperatures, albeit with a weaker rhythmicity, with just one of the proteins present.
White collar-1 (WC-1) and white collar-2 (WC-2) are GATA transcription factors in Neurospora and they form a heterodimeric "white collar complex" (WCC) via their PAS domains. When WCC is hypophosphorylated during subjective night, it binds to the frequency (frq) gene promotor and activates frq transcription. The frequency (FRQ) protein accumulates and is progressively phosphorylated by casein kinase 2 (CKII), reaching its peak around mid-subjective day. When FRQ is hyperphosphorylated, it is ubiquitinated and degraded by the proteasome. FRQ recruits kinases such as casein kinase 1a (CK-1a) that phosphorylates WCC. Hyperphosphorylated WCC is inactive and binds poorly to frq promotor. This results in inhibited frq gene transcription and a lower FRQ protein level, and eventually repression is relieved to allow frq transcription to build up again. This process occurs with a periodicity of around 22 hours in constant conditions.
Genetic analysis of the locus has identified alleles that alter the period length of asexual spore formation (conidiation). Mutations have been identified that cause both long and short periods relative to the wild-type value of 21.5 hours. Mutants attain a shorter or longer period due to the rate at which they are phosphorylated. Mutants that can reduce phosphorylation, due to a mutation in a binding site, will not degrade as rapidly and have a longer period. The reverse is true for mutants with a short period. The period of the frq mutants at 25oC are as follows: frq-1 : 16.5 hrs, frq-2: 19.3 hrs; frq-3: 24.0 hrs, frq -4: 19.3 hrs, frq-6: 19.2 hrs, frq 7: 29.0 hrs, frq-8: 29 hrs. The first three mutations are very closely linked since no genetic recombination among them is detected. Each mutant segregates as a single nuclear gene. In addition, one recessive allele, frq-9 results in conditional arrhythmicity and a complete loss of temperature compensation.
FRQ-less Oscillator (FLO)
In frq null mutants Neurospora crassa, a rhythm of conidiospore development was still observed in constant darkness (DD). The period for frq null mutants varied from 12 to 35 hours, and it could be stabilized by adding farnesol or geraniol (two intermediates of the sterol synthesis pathway) to the growth medium. In addition, although FRQ-less rhythm lost certain CLOCK characteristics such as temperature compensation, it could be reset by temperature pulses. Therefore, there was evidence for a FRQ-less oscillator in Neurospora crassa. The mechanism and significance for FRQ-less oscillator (FLO) are still under research.
Besides white collar complex (WCC), FRQ protein has also been shown to interact with FRH (FRQ-interacting RNA helicase; an essential DEAD box-containing RNA helicase in Neurospora) to form a FRQ/FRH complex(FFC).
Since codon optimization of the frq gene results in impaired circadian feedback loop function, frq displays non-optimal codon usage bias across its open reading frame in contrast to most other genes.
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