Plant Molecular Biology 39: 1175–1188, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
Genetic deletion of proteins resembling Type IV pilins in Synechocystis sp.
PCC 6803: their role in binding or transfer of newly synthesized
Qingfang He and Wim Vermaas
Department of Plant Biology and Center for the Study of Early Events in Photosynthesis, Arizona State University,
Box 871601, Tempe, AZ 85287-1601, USA (
author for correspondence)
Received 20 May 1998; accepted in revised form 25 November 1998
Key words: chlorophyllbiosynthesis, chlorophyll-bindingprotein, photosystem II, pilin-like protein, Synechocystis
Upon non-denaturing gel electrophoresis of Synechocystis sp. PCC 6803 thylakoid extracts, a Type IV pilin-like
protein encoded by open reading frame sll1694 was found in chlorophyll-containingbands. The Synechocystis sp.
PCC 6803 genome also encodes two similar open reading frames, sll1695 and slr1456. Even though transcripts of
sll1694and slr1456couldbe detected, deletion of thethreeopenreadingframes in systemswith normalchlorophyll
content had no effect. However, Sll1694 was found to affect the rate of chlorophyll synthesis and of the assembly
of chlorophyll-binding proteins. In the sll1694/sll1695 deletion mutant in a PS I-less/chlL
background, which is
unable to synthesize chlorophyll in darkness, chlorophyll synthesis during the ﬁrst hours of illumination after dark
incubation was 30% slower than in the PS I-less/chlL
strain. Moreover, the biogenesis of chlorophyll-protein
complexes with a 77K chlorophyll ﬂuorescence emission maximum at 685 mm was delayed by several hours in
this mutant whereas the rate of biogenesis of photosystem II was not signiﬁcantly affected. Furthermore, results
of non-denaturing gel electrophoresis indicated that a chlorophyll-binding complex formed during the early hours
of chlorophyll synthesis was altered in stability and mobility upon deletion of the three open reading frames. We
proposethat the protein encoded by sll1694 is involved in, but is not absolutely required for, deliveringchlorophyll
to nascent photosystems and antennae.
The unicellular cyanobacterium Synechocystis sp.
PCC 6803 contains photosystems similar to those of
plants and algae [8, 30, 38]. This prokaryote is natu-
rally transformable by exogenous DNA and can grow
its entire genome has been sequenced, identiﬁng about
3200 potential genes, about half of which have no
strong similarity to other known genes . These
properties render the organism suitable for genetic
studies of photosynthesis and other cellular processes.
Synechocystis sp. PCC 6803 synthesizes only
chlorophyll a, which is non-covalently associated
with chlorophyll-binding proteins in thylakoid mem-
branes. In plants, chlorophyll synthesis takes place in
the chloroplast, which is thought to have developed
from cyanobacteria. The set of reactions converting
protoporphyrin IX and producing protochlorophyllide
(PChlide) was suggested to occur in the chloroplast
envelope [19, 20, 33, 34]. The light-dependent re-
duction of PChlide may also occur on the chloroplast
envelope and in the cyanobacterial cytoplasmic mem-
brane as both the substrate (PChlide) and enzymatic
activity were located there rather than on the thylakoid
membrane [16, 19, 31, 32, 34]. However, the location
of esteriﬁcation of chlorophyllide with phytol as well
as the oxygenation of chlorophyll a to form chloro-
phyll b in chlorophyll b-containing systems still is
uncertain, but may take place in thylakoid membranes
[4, 24, 43]. In any case, all chlorophyll-binding an-
tenna and reaction center proteins are located in the