Genes for RC L，M，and H Polypeptides
Dot blot（see Figure 2，Table 2）and electropherogram-replica［see Figure 3（c）and（d）］hybridizations indicated highly coordinate regulation among the genes for the subunits L，M，and H of the RC complex.As shown by dot hybridization，when the high-lightgrown cells were shifted to the dark for 1 h，about 70%of the mRNAs for RC-L，RC-M，and RC-H
Figure 2 Dot blot hybridization of R.capsulatus RNA with cloned probes for LH（Ⅰ，Ⅱ），RC（L，M，H）and Bchl（BamE and-D）Crt（BamJ and-H）biosynthetic enzymes.Cells were grown under various growth conditions as indicated at left（HL，high light；D，shift from HL to dark for 1 h；LL，low light；O2，highly aerobic growth；LO2，semiaerobic growth），and RNA was isolated as described in the text.Equal amounts of RNA（5 μg）after denaturation were spotted onto GeneScreen membranes，hybridized with 32P-labeled probes，and exposed to X-ray film.Each probe is listed above the dot blots and described in Table 1
Table 2 Effects of light and O2 on mRNA levels
*The relative amounts of mRNA were measured by dot hybridization，quantified by densitometry of X-ray film，and expressed as the percentages of mRNA level under high light（set as 100）.
Figure 3 Blot hybridization of electrophoretically fractionated total RNA from R.capsulatus with cloned probes for LH-Ⅰ，LH-Ⅱ，RC-L，RC-H，and Crt biosynthetic enzymes.RNAs from R.capsulatus cells grown under various growth conditions（HL，D，LL，O2，LO2；see legend to Figure 2）were denatured in formaldehyde，fractionated in 1.2%agarose gel，transferred to nitrocellulose（a）or GeneScreen［（b）—（e）］，and hybridized with 32 P-labeled probes for LH-Ⅱ（a），LH-Ⅰ（b），RC-L（c），RC-H（d），and Crt（e）（listed in Table 1）
were degraded.Under steady-state low light，a 30%—76%increase in mRNA levels was observed.This increase agrees with the increase in mRNAs for LH-Ⅰ and LH-Ⅱ，and it is expected because more photosynthetic membrane is produced to compensate for the decreased light intensity.Steady-state aerobic cells（high O2）showed a 70%reduction in mRNA compared to high-light-grown cells，whereas the low-O2 condition had no noticeable effect on the RC-L，RC-M，and RC-H mRNA levels.This result agrees with hybridizations of electrophoretically fractionated RNA（see Figure 3）.We found that the gene for RC-H had two major transcripts，of 1.2 and 1.4 kb.They hybridized with the probes for the internal region of the gene for RC-H（T210）and with upstream regions coding for F1696（H227）（see Figure 1）.There was no hybridization with the downstream segment encoding F3981in BamHI-K（data not shown）.Therefore，we assume that the mRNA for RC-H initiates within the coding region designated ORF F1696.Although the amounts of the 1.2-and 1.4-kb transcripts were greatly reduced in dark or high-aeration growth conditions，they were regulated differently by light intensity.Under high light，the 1.2-kb transcript is more abundant than the 1.4-kb transcript，with a 1.2-kb/1.4-kb ratio of 2.5；under low light the 1.4-kb transcript predominates，with a 1.2-kb/1.4-kb ratio of 0.5.
