Mbb10

TRP OPERON & ATTENUATION

if there’s a lot of tryptophan, it follows that there are a lot of trp-carrying

tRNA available (since transcription and translation occur simultaneously in

prokaryotes). So, the ribosome pauses at the stop codons between area 1

and 2, which prevents formation of the 2-3 stem loop. the 3-4 termination

hairpin forms, so it’s “attenuated.”

attenuate: to reduce the effect, force, or value of

conversely, small amounts of trp = not many trp-carrying tRNA. so, before

the ribosome even gets to the stop codons, it stalls on the trp codons. this

gives enough time for the 2-3 hairpin to form, which means that there is no

3-4 termination hairpin—transcription proceeds into the trp structural

genes.

tl;dr

note that the most stable secondary stem-loop structure is 1-2, 3-4.

no trp: stalls on trp —> enough time for 2-3 to form —> ribosome proceeds

into structural genes.

enough trp available: ribosome does not stall on trp —> 3-4 termination

loop.

CELL TYPE-SPECIFIC MATING IN YEAST

a (haploid, mates with α)

α (haploid, mates with a)

a/α (diploid, does not mate) — reproduces by cell division

*a produces pheromones specific to it, detected by α and vice-versa

mating: i hope i don’t have to define this, bc i won’t

**my drawings for this are terrible so refer to the link in the tweet

MAT genes of a: —[a1]—

MAT genes of α: —[α2]—[α1]—

MAT genes of a/α: —[a1]—

—[α2]—[α1]—

regulatory proteins

regulatory proteins

regulatory proteins

*MCM1 is produced by a different locus

*the genes at the MAT locus determine the yeast “type”

MCM1, a1

MCM1, α2, α1

MCM1, a1, α2

— a-specific genes

MCM1 has high affinity for the promoter region. (this has something to do

with the exact sequence of the promoter. we don’t need to know the

details.)

— α-specific genes

MCM1 has low affinity for the promoter region of α-specific genes.

—haploid-specific genes

you know how eukaryotic genes are off by default? this is one of those...